<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e2cc5-d4bd-730e-a5aa-3691b2b29c8b,g:35507c1b-8705-4442-914c-13972467f32d-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.swgas.com/role/Cover" xlink:type="simple" xlink:href="swx-20260630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/Cover" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitiesTable_019e2cc5-d711-7813-bbe3-3768ecc589f1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntitiesTable_019e2cc5-d711-7813-bbe3-3768ecc589f1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-7691-bb08-4404c359e335" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_019e2cc5-d711-7813-bbe3-3768ecc589f1" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-7691-bb08-4404c359e335" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7691-bb08-4404c359e335_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7691-bb08-4404c359e335" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7691-bb08-4404c359e335_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-78cd-8693-18c236aaf152" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7691-bb08-4404c359e335" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-78cd-8693-18c236aaf152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7e2d-9706-07fb4e2aa43d" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-78cd-8693-18c236aaf152" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7e2d-9706-07fb4e2aa43d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019e2cc5-d711-7154-b049-78eb50d2174a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentType_019e2cc5-d711-7154-b049-78eb50d2174a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_019e2cc5-d711-7524-b3ec-b47da2e40987" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentQuarterlyReport_019e2cc5-d711-7524-b3ec-b47da2e40987" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019e2cc5-d711-76e7-a514-5741ecf388a9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentPeriodEndDate_019e2cc5-d711-76e7-a514-5741ecf388a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_019e2cc5-d711-735c-b1ab-ec6a56bd5c04" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentTransitionReport_019e2cc5-d711-735c-b1ab-ec6a56bd5c04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019e2cc5-d711-7e8e-b508-a62a1a47dda4" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityFileNumber_019e2cc5-d711-7e8e-b508-a62a1a47dda4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019e2cc5-d711-7cd5-9367-c1d423b66274" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityRegistrantName_019e2cc5-d711-7cd5-9367-c1d423b66274" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019e2cc5-d711-7de1-b1db-dfec5687ab65" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019e2cc5-d711-7de1-b1db-dfec5687ab65" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019e2cc5-d711-786a-af78-5784796b038e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityTaxIdentificationNumber_019e2cc5-d711-786a-af78-5784796b038e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019e2cc5-d711-78c9-a423-ed02db0e1525" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityAddressAddressLine1_019e2cc5-d711-78c9-a423-ed02db0e1525" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019e2cc5-d711-7a38-855d-63f070d9f63f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityAddressCityOrTown_019e2cc5-d711-7a38-855d-63f070d9f63f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019e2cc5-d711-79a5-8f83-9339503d027c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityAddressStateOrProvince_019e2cc5-d711-79a5-8f83-9339503d027c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019e2cc5-d711-7f55-9200-c29d030b8b52" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityAddressPostalZipCode_019e2cc5-d711-7f55-9200-c29d030b8b52" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019e2cc5-d711-737f-9d6f-d1eb323aeb1f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_CityAreaCode_019e2cc5-d711-737f-9d6f-d1eb323aeb1f" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019e2cc5-d711-79b2-8c01-7a6f9a6334d2" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_LocalPhoneNumber_019e2cc5-d711-79b2-8c01-7a6f9a6334d2" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019e2cc5-d711-7d95-946f-22bbe227c5af" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_Security12bTitle_019e2cc5-d711-7d95-946f-22bbe227c5af" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019e2cc5-d711-710d-be32-68c5e327b736" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_TradingSymbol_019e2cc5-d711-710d-be32-68c5e327b736" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019e2cc5-d711-748e-8944-4b9d35ea9b8f" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_SecurityExchangeName_019e2cc5-d711-748e-8944-4b9d35ea9b8f" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_019e2cc5-d711-7ba9-9cd9-be3f3ab5d23a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityCurrentReportingStatus_019e2cc5-d711-7ba9-9cd9-be3f3ab5d23a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_019e2cc5-d711-7f10-8eff-7deb147a2f40" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityInteractiveDataCurrent_019e2cc5-d711-7f10-8eff-7deb147a2f40" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_019e2cc5-d711-7c3c-b137-112a08405b53" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityFilerCategory_019e2cc5-d711-7c3c-b137-112a08405b53" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_019e2cc5-d711-7303-89f4-829cb53993f9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntitySmallBusiness_019e2cc5-d711-7303-89f4-829cb53993f9" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019e2cc5-d711-7e8e-965a-adff6aef4f4e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityEmergingGrowthCompany_019e2cc5-d711-7e8e-965a-adff6aef4f4e" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_019e2cc5-d711-789e-9cb0-f3ecb79e0088" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityShellCompany_019e2cc5-d711-789e-9cb0-f3ecb79e0088" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_019e2cc5-d711-7cf7-af66-9a3eafd49e3b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_019e2cc5-d711-7cf7-af66-9a3eafd49e3b" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019e2cc5-d711-7605-bdbd-601dafa0a54b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_EntityCentralIndexKey_019e2cc5-d711-7605-bdbd-601dafa0a54b" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_019e2cc5-d711-7cbc-b20e-d78a6a18adee" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentFiscalYearFocus_019e2cc5-d711-7cbc-b20e-d78a6a18adee" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_019e2cc5-d711-7059-8e23-ef7431e34578" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_DocumentFiscalPeriodFocus_019e2cc5-d711-7059-8e23-ef7431e34578" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019e2cc5-d711-711a-88be-07f8eaa216ed" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_AmendmentFlag_019e2cc5-d711-711a-88be-07f8eaa216ed" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_019e2cc5-d711-7ecd-a2a9-35064460e6f2" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019e2cc5-d711-7554-88e2-24af98212985" xlink:to="loc_dei_CurrentFiscalYearEndDate_019e2cc5-d711-7ecd-a2a9-35064460e6f2" xlink:type="arc" order="28"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7d12-ac0c-a8fb66cffccb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7d12-ac0c-a8fb66cffccb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019e2cc5-d711-7d52-91d2-5a202d4a4799" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7d12-ac0c-a8fb66cffccb" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019e2cc5-d711-7d52-91d2-5a202d4a4799" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7d52-91d2-5a202d4a4799_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019e2cc5-d711-7d52-91d2-5a202d4a4799" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7d52-91d2-5a202d4a4799_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7451-9ba8-cac0a99789eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019e2cc5-d711-7d52-91d2-5a202d4a4799" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7451-9ba8-cac0a99789eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_019e2cc5-d711-7c62-bf17-1c2f07b150d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentContinuingOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7451-9ba8-cac0a99789eb" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_019e2cc5-d711-7c62-bf17-1c2f07b150d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDiscontinuedOperationsMember_019e2cc5-d711-78de-85b7-b5cb1b63bc27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_019e2cc5-d711-7451-9ba8-cac0a99789eb" xlink:to="loc_us-gaap_SegmentDiscontinuedOperationsMember_019e2cc5-d711-78de-85b7-b5cb1b63bc27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e2cc5-d711-7438-b5ac-b2b0f079b40b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_ProfitLoss_019e2cc5-d711-7438-b5ac-b2b0f079b40b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019e2cc5-d711-7f34-a193-c0dc8c2cf393" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019e2cc5-d711-7f34-a193-c0dc8c2cf393" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-7222-94ee-d294cd4217dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-7222-94ee-d294cd4217dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-7dea-a6e7-e7cdb3292758" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-7dea-a6e7-e7cdb3292758" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d711-785e-99b4-48624cb78ec9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d711-785e-99b4-48624cb78ec9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019e4600-52f3-7212-9b24-ca89db11ab24" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019e4600-52f3-7212-9b24-ca89db11ab24" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019e2cc5-d711-76d5-8582-a12b265fcad3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_ShareBasedCompensation_019e2cc5-d711-76d5-8582-a12b265fcad3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction_019e2cc5-d711-7090-a5b2-d5dc5d49dfde" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction_019e2cc5-d711-7090-a5b2-d5dc5d49dfde" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7305-8dfa-82a919d551ad" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d711-7d46-bff3-b0b07c3de460" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d711-7d46-bff3-b0b07c3de460" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IncreaseDecreaseInAccruedUtilityRevenue_019e2cc5-d711-79c9-bb68-7a1e0d7a7e58" xlink:href="swx-20260630.xsd#swx_IncreaseDecreaseInAccruedUtilityRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_swx_IncreaseDecreaseInAccruedUtilityRevenue_019e2cc5-d711-79c9-bb68-7a1e0d7a7e58" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d711-730d-ac31-1621f8a7e178" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredGasCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d711-730d-ac31-1621f8a7e178" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d711-7621-92aa-fc0309aaef34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d711-7621-92aa-fc0309aaef34" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019e2cc5-d711-7893-bcd4-fd51032d900c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedTaxesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019e2cc5-d711-7893-bcd4-fd51032d900c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019e2cc5-d711-7c1c-8a98-3f67408ca238" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7457-b351-9bc8f577bd86" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019e2cc5-d711-7c1c-8a98-3f67408ca238" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019e2cc5-d711-7b12-bf40-0ff480e2de38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019e2cc5-d711-7b12-bf40-0ff480e2de38" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019e2cc5-d711-7807-89f5-8f1d6ac2c86a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019e2cc5-d711-7807-89f5-8f1d6ac2c86a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations_019e2cc5-d711-7de5-b5b7-e8c5677c09ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations_019e2cc5-d711-7de5-b5b7-e8c5677c09ab" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations_019e2cc5-d711-7e8d-ac53-299dfa6bad2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations_019e2cc5-d711-7e8d-ac53-299dfa6bad2f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d711-704e-8b3e-015a1116fbd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eaa-a0bc-4c1168fea906" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d711-704e-8b3e-015a1116fbd8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019e2cc5-d711-7c4a-9c57-cdab91a25407" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019e2cc5-d711-7c4a-9c57-cdab91a25407" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019e4600-52f3-7071-a926-95894b6492c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019e4600-52f3-7071-a926-95894b6492c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_ChangesInCustomerAdvancesInvestingActivities_019e2cc5-d711-7c66-87ef-1d769945b546" xlink:href="swx-20260630.xsd#swx_ChangesInCustomerAdvancesInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_swx_ChangesInCustomerAdvancesInvestingActivities_019e2cc5-d711-7c66-87ef-1d769945b546" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019e2cc5-d711-7c05-bf2b-04454bd360da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForProceedsFromOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019e2cc5-d711-7c05-bf2b-04454bd360da" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_019e2cc5-d711-7538-b391-fd4c47b30304" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_019e2cc5-d711-7538-b391-fd4c47b30304" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations_019e2cc5-d711-7874-9ea1-9bdae88d7adb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations_019e2cc5-d711-7874-9ea1-9bdae88d7adb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019e2cc5-d711-79cd-a865-761f249c6863" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7981-b2d7-32312b3bdb64" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019e2cc5-d711-79cd-a865-761f249c6863" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019e2cc5-d711-7c35-94a9-cb9d16facdec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019e2cc5-d711-7c35-94a9-cb9d16facdec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividendsCommonStock_019e2cc5-d711-7adb-986c-67a2c831e4cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_PaymentsOfDividendsCommonStock_019e2cc5-d711-7adb-986c-67a2c831e4cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromRepaymentsOfLinesOfCredit_019e2cc5-d711-7ff0-be34-c4acb327c2b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromRepaymentsOfLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_ProceedsFromRepaymentsOfLinesOfCredit_019e2cc5-d711-7ff0-be34-c4acb327c2b4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfShortTermDebt_019e4600-52f3-7203-9b96-40f1961d878d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfShortTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_RepaymentsOfShortTermDebt_019e4600-52f3-7203-9b96-40f1961d878d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019e2cc5-d711-7c06-8217-d8f1fed4554e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019e2cc5-d711-7c06-8217-d8f1fed4554e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019e2cc5-d711-758c-8e00-15901cec80c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromPaymentsForOtherFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019e2cc5-d711-758c-8e00-15901cec80c8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_019e2cc5-d711-7440-bd3b-af7fbd1dc9a0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_019e2cc5-d711-7440-bd3b-af7fbd1dc9a0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations_019e2cc5-d711-765d-8e2e-eb3e492beb82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations_019e2cc5-d711-765d-8e2e-eb3e492beb82" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019e2cc5-d711-7c22-854c-212febf8d854" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-798f-b199-700aace7644c" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019e2cc5-d711-7c22-854c-212febf8d854" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsDisposalGroupIncludingDiscontinuedOperations_019e2cc5-d711-7369-968d-b504ba842584" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsDisposalGroupIncludingDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsDisposalGroupIncludingDiscontinuedOperations_019e2cc5-d711-7369-968d-b504ba842584" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019e2cc5-d711-7a96-a4f3-325fbb688587" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019e2cc5-d711-7a96-a4f3-325fbb688587" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019e2cc5-d711-774f-b8b8-1eb55678adf6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019e2cc5-d711-774f-b8b8-1eb55678adf6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_019e2cc5-d711-7993-9786-b5d74977f036" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-74be-b37c-9a55fa4fb6db" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_019e2cc5-d711-7aaa-bf54-9771bc7d243e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:to="loc_us-gaap_InterestPaidNet_019e2cc5-d711-7aaa-bf54-9771bc7d243e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019e2cc5-d711-7f52-8413-f78fa8e2ffdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019e2cc5-d711-7f52-8413-f78fa8e2ffdc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019e2cc5-d711-7129-8d6b-20c70ed8380c" xlink:href="swx-20260630.xsd#swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:to="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019e2cc5-d711-7129-8d6b-20c70ed8380c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d711-7cf7-8a99-0221b0ea6f9c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7722-bf81-38fbfde2627f" xlink:to="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d711-7cf7-8a99-0221b0ea6f9c" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-7e8b-9916-d4489988ceaf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-71ab-be46-dbc1e919973a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7e8b-9916-d4489988ceaf" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-71ab-be46-dbc1e919973a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-7768-9479-315719103db3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-71ab-be46-dbc1e919973a" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-7768-9479-315719103db3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7768-9479-315719103db3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-7768-9479-315719103db3" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7768-9479-315719103db3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7553-aa7e-31fe5dba2eff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-7768-9479-315719103db3" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7553-aa7e-31fe5dba2eff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7553-aa7e-31fe5dba2eff" xlink:to="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e2cc5-d711-77bf-9d63-dab2867fa4ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:to="loc_us-gaap_CommonStockMember_019e2cc5-d711-77bf-9d63-dab2867fa4ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e2cc5-d711-7582-8b9b-eeb86f450f92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e2cc5-d711-7582-8b9b-eeb86f450f92" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e2cc5-d711-7256-a843-82ba2ae87430" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e2cc5-d711-7256-a843-82ba2ae87430" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e2cc5-d711-7a76-b27d-2b5c677a1ca7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ParentMember_019e2cc5-d711-7e60-8042-49f3b52fed9e" xlink:to="loc_us-gaap_RetainedEarningsMember_019e2cc5-d711-7a76-b27d-2b5c677a1ca7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_019e2cc5-d711-75ac-b766-aa61d67bd2df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7553-aa7e-31fe5dba2eff" xlink:to="loc_us-gaap_NoncontrollingInterestMember_019e2cc5-d711-75ac-b766-aa61d67bd2df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7e8b-9916-d4489988ceaf" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7503-8fc7-1191c6653c5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7503-8fc7-1191c6653c5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-78a3-a6c1-09f8a632a87f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-78a3-a6c1-09f8a632a87f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d711-7df0-a15b-6a516b26cde8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d711-7df0-a15b-6a516b26cde8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019e2cc5-d711-7492-9c44-de9d12232c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019e2cc5-d711-7492-9c44-de9d12232c9d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019e2cc5-d711-7280-9ef2-708da0b9e1a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019e2cc5-d711-7280-9ef2-708da0b9e1a3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019e2cc5-d711-7a28-9e89-234a2ff4a911" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_DividendsCommonStock_019e2cc5-d711-7a28-9e89-234a2ff4a911" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest_019e4616-cea1-7197-b43a-35ea14b21114" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_NoncontrollingInterestIncreaseFromSaleOfParentEquityInterest_019e4616-cea1-7197-b43a-35ea14b21114" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentBeforeTaxPortionAttributableToParent_019e2cc5-d711-745f-897a-c7555f9837b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentBeforeTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationAdjustmentBeforeTaxPortionAttributableToParent_019e2cc5-d711-745f-897a-c7555f9837b8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther_019e2cc5-d711-7c41-aad0-361a79f13398" xlink:href="swx-20260630.xsd#swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-788e-b4fc-93829d06623b" xlink:to="loc_swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther_019e2cc5-d711-7c41-aad0-361a79f13398" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7d78-b933-7adb2caf9e39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-77c2-ae47-be5fa8114143" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSSouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSSouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSSouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-770e-b3ee-bff307f38249" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7148-a73b-a1f1a9a64a14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-770e-b3ee-bff307f38249" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7148-a73b-a1f1a9a64a14" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-744d-bdbe-3164470e5c6b" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7148-a73b-a1f1a9a64a14" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-744d-bdbe-3164470e5c6b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-744d-bdbe-3164470e5c6b_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-744d-bdbe-3164470e5c6b" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-744d-bdbe-3164470e5c6b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7fe2-b65c-029cc56a3212" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-744d-bdbe-3164470e5c6b" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7fe2-b65c-029cc56a3212" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7ec0-8f9d-d26c4d0e3749" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7fe2-b65c-029cc56a3212" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7ec0-8f9d-d26c4d0e3749" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-7b04-abf7-c795dc1a8424" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7148-a73b-a1f1a9a64a14" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-7b04-abf7-c795dc1a8424" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b04-abf7-c795dc1a8424_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-7b04-abf7-c795dc1a8424" xlink:to="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b04-abf7-c795dc1a8424_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7010-bc7e-d42798d7bbec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-7b04-abf7-c795dc1a8424" xlink:to="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7010-bc7e-d42798d7bbec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019e2cc5-d711-713b-bd14-7a967cd92b2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7010-bc7e-d42798d7bbec" xlink:to="loc_us-gaap_NonrelatedPartyMember_019e2cc5-d711-713b-bd14-7a967cd92b2d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019e2cc5-d711-756d-91a7-1e83bed4f5d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7010-bc7e-d42798d7bbec" xlink:to="loc_us-gaap_RelatedPartyMember_019e2cc5-d711-756d-91a7-1e83bed4f5d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-770e-b3ee-bff307f38249" xlink:to="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_PublicUtilitiesPropertyPlantAndEquipmentGasPlant_019e2cc5-d711-79d9-b5d1-2f06798a5bb8" xlink:href="swx-20260630.xsd#swx_PublicUtilitiesPropertyPlantAndEquipmentGasPlant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:to="loc_swx_PublicUtilitiesPropertyPlantAndEquipmentGasPlant_019e2cc5-d711-79d9-b5d1-2f06798a5bb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation_019e2cc5-d711-7bf0-aea8-f696dfba1a38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation_019e2cc5-d711-7bf0-aea8-f696dfba1a38" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress_019e2cc5-d711-7712-a968-a482b5077a94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress_019e2cc5-d711-7712-a968-a482b5077a94" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_019e2cc5-d711-76c0-a0b6-4ae1f214d96f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNetAbstract_019e2cc5-d711-7d91-8f5e-5dc2a229be14" xlink:to="loc_us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet_019e2cc5-d711-76c0-a0b6-4ae1f214d96f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherPropertyAndInvestments_019e2cc5-d711-7a8f-9703-bc39407c2e22" xlink:href="swx-20260630.xsd#swx_OtherPropertyAndInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:to="loc_swx_OtherPropertyAndInvestments_019e2cc5-d711-7a8f-9703-bc39407c2e22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e2cc5-d711-7d01-b718-9d89bf856238" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e2cc5-d711-7d01-b718-9d89bf856238" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019e2cc5-d711-79d9-be3d-b5adffbf6f1f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019e2cc5-d711-79d9-be3d-b5adffbf6f1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AccruedUtilityRevenue_019e2cc5-d711-7f5a-bbdd-f55431881d84" xlink:href="swx-20260630.xsd#swx_AccruedUtilityRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_swx_AccruedUtilityRevenue_019e2cc5-d711-7f5a-bbdd-f55431881d84" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesReceivable_019e2cc5-d711-74f3-b3a4-d8b40d254e0b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxesReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_IncomeTaxesReceivable_019e2cc5-d711-74f3-b3a4-d8b40d254e0b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredGasCost_019e2cc5-d711-71bf-a866-1cba2c1e96df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredGasCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_DeferredGasCost_019e2cc5-d711-71bf-a866-1cba2c1e96df" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialsSuppliesAndOther_019e2cc5-d711-73f0-a0ed-8aceb082b0e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MaterialsSuppliesAndOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_MaterialsSuppliesAndOther_019e2cc5-d711-73f0-a0ed-8aceb082b0e9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019e2cc5-d711-752f-ba7e-fef9e6ac5521" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019e2cc5-d711-752f-ba7e-fef9e6ac5521" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019e2cc5-d711-7929-94e3-ced71f0b5420" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019e2cc5-d711-7237-8613-87c2515b5580" xlink:to="loc_us-gaap_AssetsCurrent_019e2cc5-d711-7929-94e3-ced71f0b5420" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsNoncurrentAbstract_019e2cc5-d711-75a4-a4dc-4713a7efa024" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsNoncurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:to="loc_us-gaap_AssetsNoncurrentAbstract_019e2cc5-d711-75a4-a4dc-4713a7efa024" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019e2cc5-d711-7ee0-85a7-daec9ef8e6c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_019e2cc5-d711-75a4-a4dc-4713a7efa024" xlink:to="loc_us-gaap_Goodwill_019e2cc5-d711-7ee0-85a7-daec9ef8e6c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019e2cc5-d711-7bcb-88b2-718f276b4c6b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_019e2cc5-d711-75a4-a4dc-4713a7efa024" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019e2cc5-d711-7bcb-88b2-718f276b4c6b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsNoncurrentOtherThanNoncurrentInvestmentsAndPropertyPlantAndEquipment_019e2cc5-d711-784d-82d5-d906fbba682f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsNoncurrentOtherThanNoncurrentInvestmentsAndPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsNoncurrentAbstract_019e2cc5-d711-75a4-a4dc-4713a7efa024" xlink:to="loc_us-gaap_AssetsNoncurrentOtherThanNoncurrentInvestmentsAndPropertyPlantAndEquipment_019e2cc5-d711-784d-82d5-d906fbba682f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019e2cc5-d711-7660-ab42-f9678b04718b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e2cc5-d711-73e5-a574-d3ec098a6693" xlink:to="loc_us-gaap_Assets_019e2cc5-d711-7660-ab42-f9678b04718b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-770e-b3ee-bff307f38249" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizationLongtermDebtAndEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:to="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_019e2cc5-d711-7b17-a04f-b5f2d361aeb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_CommonStockValue_019e2cc5-d711-7b17-a04f-b5f2d361aeb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019e2cc5-d711-7315-abdd-9a8ee93d90b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_019e2cc5-d711-7315-abdd-9a8ee93d90b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e2cc5-d711-7261-a4fe-68c1def23d3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019e2cc5-d711-7261-a4fe-68c1def23d3f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019e2cc5-d711-7553-805f-4e29f9b0b7b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019e2cc5-d711-7553-805f-4e29f9b0b7b5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e2cc5-d711-7fc9-9c8a-a93e5252ed90" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_StockholdersEquity_019e2cc5-d711-7fc9-9c8a-a93e5252ed90" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e2cc5-d711-7927-9af7-9d18f1af3517" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e2cc5-d711-7927-9af7-9d18f1af3517" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizationLongtermDebtAndEquity_019e2cc5-d711-74c9-b698-f7f7cf8d8347" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizationLongtermDebtAndEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizationLongtermDebtAndEquityAbstract_019e2cc5-d711-74c8-9327-317844b1aaef" xlink:to="loc_us-gaap_CapitalizationLongtermDebtAndEquity_019e2cc5-d711-74c9-b698-f7f7cf8d8347" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e2cc5-d711-7412-aae3-581b31d2aae2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e2cc5-d711-7412-aae3-581b31d2aae2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019e2cc5-d711-771d-8d3e-ce99d8930f52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_AccountsPayableCurrent_019e2cc5-d711-771d-8d3e-ce99d8930f52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e2cc5-d711-7111-a934-d05168e081a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_019e2cc5-d711-7111-a934-d05168e081a4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForTaxesOtherThanIncomeTaxesCurrent_019e2cc5-d711-7581-97c8-4862db0f8661" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccrualForTaxesOtherThanIncomeTaxesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_AccrualForTaxesOtherThanIncomeTaxesCurrent_019e2cc5-d711-7581-97c8-4862db0f8661" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrent_019e2cc5-d711-7b48-8615-6816039f2e52" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_InterestPayableCurrent_019e2cc5-d711-7b48-8615-6816039f2e52" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredGasPurchasesCurrent_019e2cc5-d711-7a61-836f-0a240a59b24b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredGasPurchasesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_DeferredGasPurchasesCurrent_019e2cc5-d711-7a61-836f-0a240a59b24b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_019e2cc5-d711-73c2-9009-7f27c76fe115" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_019e2cc5-d711-73c2-9009-7f27c76fe115" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019e2cc5-d711-76ad-8e99-0662957b6d38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019e2cc5-d711-7dfe-a3dc-50e3ba208ed2" xlink:to="loc_us-gaap_LiabilitiesCurrent_019e2cc5-d711-76ad-8e99-0662957b6d38" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesNoncurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DeferredIncomeTaxesAndUnamortizedInvestmentTaxCredits_019e2cc5-d711-7aa0-84b0-e191054dda88" xlink:href="swx-20260630.xsd#swx_DeferredIncomeTaxesAndUnamortizedInvestmentTaxCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:to="loc_swx_DeferredIncomeTaxesAndUnamortizedInvestmentTaxCredits_019e2cc5-d711-7aa0-84b0-e191054dda88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesNoncurrent_019e2cc5-d711-7ebf-a119-001b4e5858c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherAccruedLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesNoncurrent_019e2cc5-d711-7ebf-a119-001b4e5858c6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCreditsAndOtherLiabilitiesNoncurrent_019e2cc5-d711-78e4-8fd1-495cfcdc7bdb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredCreditsAndOtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:to="loc_us-gaap_DeferredCreditsAndOtherLiabilitiesNoncurrent_019e2cc5-d711-78e4-8fd1-495cfcdc7bdb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_019e2cc5-d711-74c8-838e-8e4b28aa8ae6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherLiabilitiesNoncurrentAbstract_019e2cc5-d711-7d63-99b6-106133066d20" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_019e2cc5-d711-74c8-838e-8e4b28aa8ae6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019e2cc5-d711-7d9c-a5c8-035fd790bf14" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019e2cc5-d711-70b5-875d-9ea57afc08a6" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019e2cc5-d711-7d9c-a5c8-035fd790bf14" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMESouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMESouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMESouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7bcf-97eb-9c8492664265" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7bcf-97eb-9c8492664265" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-74c7-8e20-57f4c8e94f99" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7bcf-97eb-9c8492664265" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-74c7-8e20-57f4c8e94f99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-74c7-8e20-57f4c8e94f99_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-74c7-8e20-57f4c8e94f99" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-74c7-8e20-57f4c8e94f99_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7687-af67-45c19eead4f9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-74c7-8e20-57f4c8e94f99" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7687-af67-45c19eead4f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7c5c-932e-1602b0a2ada9" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7687-af67-45c19eead4f9" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7c5c-932e-1602b0a2ada9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatedOperatingRevenue_019e2cc5-d711-72c4-afc5-6d9ebfdffcac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatedOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_RegulatedOperatingRevenue_019e2cc5-d711-72c4-afc5-6d9ebfdffcac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostsAndExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseGasAndPetroleumPurchased_019e2cc5-d711-704d-b146-5e5c8e4d2775" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UtilitiesOperatingExpenseGasAndPetroleumPurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseGasAndPetroleumPurchased_019e2cc5-d711-704d-b146-5e5c8e4d2775" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_019e2cc5-d711-7cca-8e66-0d5ba338e206" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_019e2cc5-d711-7cca-8e66-0d5ba338e206" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-74a3-be3c-79f1fd2390af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:to="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-74a3-be3c-79f1fd2390af" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxesExcludingIncomeAndExciseTaxes_019e2cc5-d711-71de-a712-7bbc25fc535b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TaxesExcludingIncomeAndExciseTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:to="loc_us-gaap_TaxesExcludingIncomeAndExciseTaxes_019e2cc5-d711-71de-a712-7bbc25fc535b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_019e2cc5-d711-798b-8feb-29b8d511bbbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostsAndExpensesAbstract_019e2cc5-d711-7159-8022-4d73cf5c9122" xlink:to="loc_us-gaap_CostsAndExpenses_019e2cc5-d711-798b-8feb-29b8d511bbbb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019e2cc5-d711-751a-807f-706112b7cc00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_OperatingIncomeLoss_019e2cc5-d711-751a-807f-706112b7cc00" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019e2cc5-d711-7e3e-918b-0bf977e97873" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019e2cc5-d711-7e3e-918b-0bf977e97873" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019e2cc5-d711-7cf8-85ea-3e96f120bb1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019e2cc5-d711-7e3e-918b-0bf977e97873" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019e2cc5-d711-7cf8-85ea-3e96f120bb1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e2cc5-d711-7a88-9379-91e56596589a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019e2cc5-d711-7e3e-918b-0bf977e97873" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e2cc5-d711-7a88-9379-91e56596589a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_019e2cc5-d711-76ca-b6a8-1222f0b05e87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NonoperatingIncomeExpenseAbstract_019e2cc5-d711-7e3e-918b-0bf977e97873" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_019e2cc5-d711-76ca-b6a8-1222f0b05e87" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e2cc5-d711-7e17-85bb-6748756402fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e2cc5-d711-7e17-85bb-6748756402fc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e2cc5-d711-7d3f-b509-ce3958328753" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e2cc5-d711-7d3f-b509-ce3958328753" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e2cc5-d711-7ee8-b672-b3126ea26ae2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-72ea-be16-bcb33b8955f0" xlink:to="loc_us-gaap_NetIncomeLoss_019e2cc5-d711-7ee8-b672-b3126ea26ae2" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMESouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMESouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMESouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-71dc-9f95-2eff3ada32d5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7e0f-a220-5f9748729e4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-71dc-9f95-2eff3ada32d5" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7e0f-a220-5f9748729e4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-7b8c-9f15-31e8214ba1f6" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7e0f-a220-5f9748729e4e" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-7b8c-9f15-31e8214ba1f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7b8c-9f15-31e8214ba1f6_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7b8c-9f15-31e8214ba1f6" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7b8c-9f15-31e8214ba1f6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7332-b487-094997f2798d" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7b8c-9f15-31e8214ba1f6" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7332-b487-094997f2798d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7778-9ba3-f4b9515ce434" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7332-b487-094997f2798d" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7778-9ba3-f4b9515ce434" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e2cc5-d711-72c1-befe-889d06149b25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-71dc-9f95-2eff3ada32d5" xlink:to="loc_us-gaap_ProfitLoss_019e2cc5-d711-72c1-befe-889d06149b25" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_019e2cc5-d711-71ad-a19f-02d5cd10d1f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-71dc-9f95-2eff3ada32d5" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_019e2cc5-d711-71ad-a19f-02d5cd10d1f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_019e2cc5-d711-71ad-a19f-02d5cd10d1f6" xlink:to="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax_019e2cc5-d711-7e40-959c-fd82d381baec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax_019e2cc5-d711-7e40-959c-fd82d381baec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_019e2cc5-d711-7e03-a9e4-5821378ca211" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax_019e2cc5-d711-7e03-a9e4-5821378ca211" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentNetOfTax_019e2cc5-d711-7d08-b4a5-5fe3de9b65b6" xlink:href="swx-20260630.xsd#swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:to="loc_swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentNetOfTax_019e2cc5-d711-7d08-b4a5-5fe3de9b65b6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax_019e2cc5-d711-7297-bb0d-c8a69510108a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecreaseAbstract_019e2cc5-d711-788b-a7d5-3ac8b1b5662a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax_019e2cc5-d711-7297-bb0d-c8a69510108a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e2cc5-d711-7371-85e4-eef45f440546" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract_019e2cc5-d711-71ad-a19f-02d5cd10d1f6" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019e2cc5-d711-7371-85e4-eef45f440546" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-71b0-af0e-5d00a31c03a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-71dc-9f95-2eff3ada32d5" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-71b0-af0e-5d00a31c03a1" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEParentheticalSouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEParentheticalSouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEParentheticalSouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-711a-827f-61e6b251f1da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7ded-9616-f87d4de4dda6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-711a-827f-61e6b251f1da" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7ded-9616-f87d4de4dda6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-7134-a8db-f7bd8aae6425" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7ded-9616-f87d4de4dda6" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-7134-a8db-f7bd8aae6425" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7134-a8db-f7bd8aae6425_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7134-a8db-f7bd8aae6425" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7134-a8db-f7bd8aae6425_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7f5a-b529-cf35d8b8c490" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7134-a8db-f7bd8aae6425" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7f5a-b529-cf35d8b8c490" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7b6f-9e69-38053001dfb2" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7f5a-b529-cf35d8b8c490" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7b6f-9e69-38053001dfb2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax_019e2cc5-d711-7a8c-a4ef-5759e77c9285" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-711a-827f-61e6b251f1da" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditTax_019e2cc5-d711-7a8c-a4ef-5759e77c9285" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax_019e2cc5-d711-7bef-9864-31069b8b5eaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-711a-827f-61e6b251f1da" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossTax_019e2cc5-d711-7bef-9864-31069b8b5eaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentTax_019e2cc5-d711-7929-bf7f-60c375990adc" xlink:href="swx-20260630.xsd#swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-711a-827f-61e6b251f1da" xlink:to="loc_swx_OtherComprehensiveIncomeDefinedBenefitPlansRegulatoryAdjustmentTax_019e2cc5-d711-7929-bf7f-60c375990adc" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSSouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSSouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSSouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-71b1-87cd-dfed6dbcfff9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-71b1-87cd-dfed6dbcfff9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-7060-ae75-c2d27cd855a9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-71b1-87cd-dfed6dbcfff9" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-7060-ae75-c2d27cd855a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7060-ae75-c2d27cd855a9_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7060-ae75-c2d27cd855a9" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7060-ae75-c2d27cd855a9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7f86-9c6d-913bd42bfcbe" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7060-ae75-c2d27cd855a9" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7f86-9c6d-913bd42bfcbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7b22-90f8-e397af8bef94" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7f86-9c6d-913bd42bfcbe" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7b22-90f8-e397af8bef94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-72b7-89fd-ed4d13fa2037" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-71b1-87cd-dfed6dbcfff9" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-72b7-89fd-ed4d13fa2037" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-72b7-89fd-ed4d13fa2037_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-72b7-89fd-ed4d13fa2037" xlink:to="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-72b7-89fd-ed4d13fa2037_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b2e-b67f-3ee1c49201f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e2cc5-d711-72b7-89fd-ed4d13fa2037" xlink:to="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b2e-b67f-3ee1c49201f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019e2cc5-d711-7643-a196-52ad52dc9d60" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b2e-b67f-3ee1c49201f1" xlink:to="loc_us-gaap_NonrelatedPartyMember_019e2cc5-d711-7643-a196-52ad52dc9d60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019e2cc5-d711-7de8-b36f-6471efb43738" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e2cc5-d711-7b2e-b67f-3ee1c49201f1" xlink:to="loc_us-gaap_RelatedPartyMember_019e2cc5-d711-7de8-b36f-6471efb43738" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e2cc5-d711-7542-bafe-2f49f428df58" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:to="loc_us-gaap_ProfitLoss_019e2cc5-d711-7542-bafe-2f49f428df58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-7976-bea0-794ffbe771a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_DepreciationAndAmortization_019e2cc5-d711-7976-bea0-794ffbe771a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d711-7328-9691-fdb1fe61585a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d711-7328-9691-fdb1fe61585a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019e49cf-e229-7144-b3e9-2e14805f5353" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019e49cf-e229-7144-b3e9-2e14805f5353" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019e2cc5-d711-79a9-b5eb-7576951f43d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_ShareBasedCompensation_019e2cc5-d711-79a9-b5eb-7576951f43d2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction_019e2cc5-d711-78bf-8934-960a4d92993d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_IncreaseDecreaseInAllowanceForEquityFundsUsedDuringConstruction_019e2cc5-d711-78bf-8934-960a4d92993d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-77eb-ad6e-b0c7f257ac46" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d711-7deb-af86-08c7a88b79f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d711-7deb-af86-08c7a88b79f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IncreaseDecreaseInAccruedUtilityRevenue_019e2cc5-d711-7818-aeec-b69753f95ef1" xlink:href="swx-20260630.xsd#swx_IncreaseDecreaseInAccruedUtilityRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_swx_IncreaseDecreaseInAccruedUtilityRevenue_019e2cc5-d711-7818-aeec-b69753f95ef1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d711-76bc-9882-177039742903" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredGasCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d711-76bc-9882-177039742903" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d711-73c5-b5d7-c30720d28b04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d711-73c5-b5d7-c30720d28b04" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019e2cc5-d711-7b8a-a2f6-dd634e643f80" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedTaxesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019e2cc5-d711-7b8a-a2f6-dd634e643f80" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019e2cc5-d711-7b2a-8822-fcec43d51239" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d711-7f0e-b71e-3896383ee974" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019e2cc5-d711-7b2a-8822-fcec43d51239" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019e2cc5-d711-7c1f-8625-3ae5d4d3005a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019e2cc5-d711-7c1f-8625-3ae5d4d3005a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019e2cc5-d711-7324-95bd-a8e0412eb148" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019e2cc5-d711-7324-95bd-a8e0412eb148" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d711-77c2-9d79-60aca8dadff1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d711-7eb0-9e8f-65843c0e9e92" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d711-77c2-9d79-60aca8dadff1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019e2cc5-d711-7a31-9a6c-53f395c7c343" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019e2cc5-d711-7a31-9a6c-53f395c7c343" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019e49cf-e229-7e0f-9be4-65b9cdd83d39" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:to="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_019e49cf-e229-7e0f-9be4-65b9cdd83d39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_ChangesInCustomerAdvancesInvestingActivities_019e2cc5-d711-7a7f-bb6d-2395e40f16d8" xlink:href="swx-20260630.xsd#swx_ChangesInCustomerAdvancesInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:to="loc_swx_ChangesInCustomerAdvancesInvestingActivities_019e2cc5-d711-7a7f-bb6d-2395e40f16d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019e2cc5-d711-76aa-a7f7-fdf23ed9899a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForProceedsFromOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:to="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019e2cc5-d711-76aa-a7f7-fdf23ed9899a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019e2cc5-d711-7a83-8ac8-6261907ced35" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019e2cc5-d711-7b40-ac54-62db43de23d4" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019e2cc5-d711-7a83-8ac8-6261907ced35" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromContributionsFromParent_019e49cf-e229-760c-9ca5-581f74bb2ea5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromContributionsFromParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:to="loc_us-gaap_ProceedsFromContributionsFromParent_019e49cf-e229-760c-9ca5-581f74bb2ea5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDividendsCommonStock_019e2cc5-d711-7931-a1f9-0e02c1fa385d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:to="loc_us-gaap_PaymentsOfDividendsCommonStock_019e2cc5-d711-7931-a1f9-0e02c1fa385d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019e2cc5-d711-7ac1-a0a8-ddae1a57705e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019e2cc5-d711-7ac1-a0a8-ddae1a57705e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019e2cc5-d711-72b1-a84a-a6c29193c19d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromPaymentsForOtherFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:to="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019e2cc5-d711-72b1-a84a-a6c29193c19d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019e2cc5-d711-7db4-baa3-7b1ea5627555" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019e2cc5-d711-728d-ac36-09e961678e40" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019e2cc5-d711-7db4-baa3-7b1ea5627555" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019e2cc5-d711-7f03-8e75-10da1dde85a8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_019e2cc5-d711-7f03-8e75-10da1dde85a8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019e2cc5-d711-70f5-a093-dc5f53b04b31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019e2cc5-d711-70f5-a093-dc5f53b04b31" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_019e2cc5-d711-7712-994c-281fa74a307b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-7ae0-840e-245107bc5e4f" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_019e2cc5-d711-7cb4-9b70-c8c412cfff06" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:to="loc_us-gaap_InterestPaidNet_019e2cc5-d711-7cb4-9b70-c8c412cfff06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019e2cc5-d711-7813-a41b-7d59504162a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019e2cc5-d711-7813-a41b-7d59504162a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019e2cc5-d711-7077-a44b-bfd575fd4a86" xlink:href="swx-20260630.xsd#swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:to="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019e2cc5-d711-7077-a44b-bfd575fd4a86" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d711-7ee4-83c0-1bc7d8ae7f01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019e2cc5-d711-7bf1-8855-09f0a790caf0" xlink:to="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d711-7ee4-83c0-1bc7d8ae7f01" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITYSouthwest" xlink:type="simple" xlink:href="swx-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITYSouthwest"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFEQUITYSouthwest" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e2cc5-d711-744c-888c-a1ff7ee6bf7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e2cc5-d711-7e06-9c8e-0f37b64a4348" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-744c-888c-a1ff7ee6bf7a" xlink:to="loc_us-gaap_StatementTable_019e2cc5-d711-7e06-9c8e-0f37b64a4348" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-70c1-8149-8575176c9f08" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7e06-9c8e-0f37b64a4348" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-70c1-8149-8575176c9f08" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-70c1-8149-8575176c9f08_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-70c1-8149-8575176c9f08" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-70c1-8149-8575176c9f08_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d711-70c1-8149-8575176c9f08" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e2cc5-d711-74a0-9034-8dfc7f9e0960" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:to="loc_us-gaap_CommonStockMember_019e2cc5-d711-74a0-9034-8dfc7f9e0960" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e2cc5-d711-7ee3-97e9-fde905bfc0e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e2cc5-d711-7ee3-97e9-fde905bfc0e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e2cc5-d711-71b0-8580-1f38966278fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e2cc5-d711-71b0-8580-1f38966278fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e2cc5-d711-7c87-8504-afddf39669e8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d711-7d51-96b1-3b843b82d783" xlink:to="loc_us-gaap_RetainedEarningsMember_019e2cc5-d711-7c87-8504-afddf39669e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d711-7245-ac62-97da9b14bf51" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e2cc5-d711-7e06-9c8e-0f37b64a4348" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d711-7245-ac62-97da9b14bf51" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7245-ac62-97da9b14bf51_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7245-ac62-97da9b14bf51" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7245-ac62-97da9b14bf51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d711-7a0c-926d-6574b85e0c64" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d711-7245-ac62-97da9b14bf51" xlink:to="loc_dei_EntityDomain_019e2cc5-d711-7a0c-926d-6574b85e0c64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7c6f-aefb-443917c1efa1" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d711-7a0c-926d-6574b85e0c64" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d711-7c6f-aefb-443917c1efa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e2cc5-d711-744c-888c-a1ff7ee6bf7a" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7b56-b5dd-f524498e3e7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7b56-b5dd-f524498e3e7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-77d4-8f34-587796e9c7c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-77d4-8f34-587796e9c7c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d711-71f2-8d56-a83cc376e7af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d711-71f2-8d56-a83cc376e7af" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_019e49d8-66d0-715d-977c-f28520e736d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_us-gaap_DividendsCommonStock_019e49d8-66d0-715d-977c-f28520e736d1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarningsNetting_019e2cc5-d711-7bb2-a6b7-a3b0f8bd0d49" xlink:href="swx-20260630.xsd#swx_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarningsNetting"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_swx_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarningsNetting_019e2cc5-d711-7bb2-a6b7-a3b0f8bd0d49" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AdjustmentsToAdditionalPaidInCapitalContributionsFromParent_019e49d6-eb12-72b3-810d-42d3b78e2d99" xlink:href="swx-20260630.xsd#swx_AdjustmentsToAdditionalPaidInCapitalContributionsFromParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_swx_AdjustmentsToAdditionalPaidInCapitalContributionsFromParent_019e49d6-eb12-72b3-810d-42d3b78e2d99" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther_019e2cc5-d711-7538-a5ab-8a27b48bc37c" xlink:href="swx-20260630.xsd#swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d711-7448-ba2a-1da54f04de81" xlink:to="loc_swx_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTaxOther_019e2cc5-d711-7538-a5ab-8a27b48bc37c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e2cc5-d711-7743-8e05-a73eb5d39612" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d711-7b9a-9d53-c78f5df7f645" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#SummaryofSignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:href="swx-20260630.xsd#swx_SignificantAccountingPoliciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:href="swx-20260630.xsd#swx_SignificantAccountingPoliciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d712-7cf4-9f84-0fb5c9722de0" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d712-7cf4-9f84-0fb5c9722de0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-7cf4-9f84-0fb5c9722de0_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7cf4-9f84-0fb5c9722de0" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-7cf4-9f84-0fb5c9722de0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-7842-a916-ea759e4d7a03" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7cf4-9f84-0fb5c9722de0" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-7842-a916-ea759e4d7a03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-79b3-95bc-ee8a3d5cfb46" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d712-7842-a916-ea759e4d7a03" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-79b3-95bc-ee8a3d5cfb46" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e2cc5-d712-7bb9-a306-3ad85091115b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e2cc5-d712-76d6-8ea8-2b12bf68c7fe" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e2cc5-d712-7bb9-a306-3ad85091115b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_PublicOfferingAndPrivatePlacementMember_019e2cc5-d712-7942-80de-36aed305fe6c" xlink:href="swx-20260630.xsd#swx_PublicOfferingAndPrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e2cc5-d712-7bb9-a306-3ad85091115b" xlink:to="loc_swx_PublicOfferingAndPrivatePlacementMember_019e2cc5-d712-7942-80de-36aed305fe6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatoryAssetAxis_019e2cc5-d712-7b5d-8348-1b71aa29b764" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatoryAssetAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:to="loc_us-gaap_RegulatoryAssetAxis_019e2cc5-d712-7b5d-8348-1b71aa29b764" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-7b5d-8348-1b71aa29b764_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatoryAssetDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RegulatoryAssetAxis_019e2cc5-d712-7b5d-8348-1b71aa29b764" xlink:to="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-7b5d-8348-1b71aa29b764_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-75ba-8225-75bafebc6a83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatoryAssetDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RegulatoryAssetAxis_019e2cc5-d712-7b5d-8348-1b71aa29b764" xlink:to="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-75ba-8225-75bafebc6a83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DecouplingMechanismMember_019e2cc5-d712-7b54-8742-a11e695bbfc7" xlink:href="swx-20260630.xsd#swx_DecouplingMechanismMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-75ba-8225-75bafebc6a83" xlink:to="loc_swx_DecouplingMechanismMember_019e2cc5-d712-7b54-8742-a11e695bbfc7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_MarginTrackerForSouthernNevadaMember_019e2cc5-d712-79e9-97ff-705b86ff6220" xlink:href="swx-20260630.xsd#swx_MarginTrackerForSouthernNevadaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-75ba-8225-75bafebc6a83" xlink:to="loc_swx_MarginTrackerForSouthernNevadaMember_019e2cc5-d712-79e9-97ff-705b86ff6220" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AccruedPensionAndOtherPostretirementBenefitCostsMember_019e2cc5-d712-725c-ac0e-1c7c428c7d90" xlink:href="swx-20260630.xsd#swx_AccruedPensionAndOtherPostretirementBenefitCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RegulatoryAssetDomain_019e2cc5-d712-75ba-8225-75bafebc6a83" xlink:to="loc_swx_AccruedPensionAndOtherPostretirementBenefitCostsMember_019e2cc5-d712-725c-ac0e-1c7c428c7d90" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7a7a-a9a9-283c44772713" xlink:to="loc_srt_LitigationCaseAxis_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7" xlink:to="loc_srt_LitigationCaseTypeDomain_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019fada5-9a1a-7be6-a0eb-7470f7fd55d3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019fada5-9a1a-7188-b0f8-1ac7f2f7bdd7" xlink:to="loc_srt_LitigationCaseTypeDomain_019fada5-9a1a-7be6-a0eb-7470f7fd55d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_CaliforniaGeneralRateCaseMember_019fada5-9a1a-76b1-8ef2-abe94cc34a91" xlink:href="swx-20260630.xsd#swx_CaliforniaGeneralRateCaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019fada5-9a1a-7be6-a0eb-7470f7fd55d3" xlink:to="loc_swx_CaliforniaGeneralRateCaseMember_019fada5-9a1a-76b1-8ef2-abe94cc34a91" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e2cc5-d712-714a-8e0c-e3ab3f3748c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e2cc5-d712-714a-8e0c-e3ab3f3748c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f9330-f54b-7d9b-bd8b-6fe2d468fd2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f9330-f54b-7d9b-bd8b-6fe2d468fd2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_019e2cc5-d712-7ca5-b5f4-6b35e43176f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_NumberOfReportableSegments_019e2cc5-d712-7ca5-b5f4-6b35e43176f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_GasPipeMaterialsAndOperatingSupplies_019e2cc5-d712-7a07-8c72-f3e43f2dc664" xlink:href="swx-20260630.xsd#swx_GasPipeMaterialsAndOperatingSupplies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_swx_GasPipeMaterialsAndOperatingSupplies_019e2cc5-d712-7a07-8c72-f3e43f2dc664" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatoryAssets_019e2cc5-d712-7cca-8e81-112884bd509f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatoryAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_RegulatoryAssets_019e2cc5-d712-7cca-8e81-112884bd509f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatedOperatingRevenue_019fada5-9a1a-72d8-9a32-856ff3bb16d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatedOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_RegulatedOperatingRevenue_019fada5-9a1a-72d8-9a32-856ff3bb16d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SoftwareCloudBased_019e2cc5-d712-7bd9-8e3a-a199f157095b" xlink:href="swx-20260630.xsd#swx_SoftwareCloudBased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_swx_SoftwareCloudBased_019e2cc5-d712-7bd9-8e3a-a199f157095b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AccruedPropertyTaxes_019e2cc5-d712-767f-9777-232688281ba7" xlink:href="swx-20260630.xsd#swx_AccruedPropertyTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_swx_AccruedPropertyTaxes_019e2cc5-d712-767f-9777-232688281ba7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019e2cc5-d712-74d7-b678-0fa909d37fd6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-72b7-b7e2-ee96cce5c46c" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019e2cc5-d712-74d7-b678-0fa909d37fd6" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesScheduleofOtherPropertyandInvestmentsDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#SummaryofSignificantAccountingPoliciesScheduleofOtherPropertyandInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesScheduleofOtherPropertyandInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e2cc5-d712-7c2b-8eaf-45697e01297d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e2cc5-d712-7c8c-8b3b-185583d82b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e2cc5-d712-7c2b-8eaf-45697e01297d" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e2cc5-d712-7c8c-8b3b-185583d82b8f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e2cc5-d712-7c8c-8b3b-185583d82b8f" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-7083-90ea-90085b2d1b6a" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7f0f-9c9e-2b0b6dbad22c" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-7083-90ea-90085b2d1b6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-7b4c-9b65-c0fcc253b9bf" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d712-7083-90ea-90085b2d1b6a" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-7b4c-9b65-c0fcc253b9bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashSurrenderValueOfLifeInsurance_019e2cc5-d712-7b0d-bfb3-77a8c8bdca27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashSurrenderValueOfLifeInsurance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e2cc5-d712-7c2b-8eaf-45697e01297d" xlink:to="loc_us-gaap_CashSurrenderValueOfLifeInsurance_019e2cc5-d712-7b0d-bfb3-77a8c8bdca27" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentOtherNet_019e2cc5-d712-7abb-9f73-ee198ab0c2aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentOtherNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e2cc5-d712-7c2b-8eaf-45697e01297d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentOtherNet_019e2cc5-d712-7abb-9f73-ee198ab0c2aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherPropertyAndInvestments_019e2cc5-d712-796c-8e78-721bc52b3934" xlink:href="swx-20260630.xsd#swx_OtherPropertyAndInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e2cc5-d712-7c2b-8eaf-45697e01297d" xlink:to="loc_swx_OtherPropertyAndInvestments_019e2cc5-d712-796c-8e78-721bc52b3934" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesScheduleofSignificantItemsIncludedinOtherIncomeDeductionsDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#SummaryofSignificantAccountingPoliciesScheduleofSignificantItemsIncludedinOtherIncomeDeductionsDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/SummaryofSignificantAccountingPoliciesScheduleofSignificantItemsIncludedinOtherIncomeDeductionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-7ebb-aaa0-735d7eae7312" xlink:href="swx-20260630.xsd#swx_SignificantAccountingPoliciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7c5d-96d0-f1ca369ca7fc" xlink:href="swx-20260630.xsd#swx_SignificantAccountingPoliciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-7ebb-aaa0-735d7eae7312" xlink:to="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7c5d-96d0-f1ca369ca7fc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d712-7b7a-833e-c2636b4acce1" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_swx_SignificantAccountingPoliciesTable_019e2cc5-d712-7c5d-96d0-f1ca369ca7fc" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d712-7b7a-833e-c2636b4acce1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-7b7a-833e-c2636b4acce1_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7b7a-833e-c2636b4acce1" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-7b7a-833e-c2636b4acce1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d712-725a-bfb7-bc30db346694" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d712-7b7a-833e-c2636b4acce1" xlink:to="loc_dei_EntityDomain_019e2cc5-d712-725a-bfb7-bc30db346694" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-7f6e-868b-79021f41e5e5" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d712-725a-bfb7-bc30db346694" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d712-7f6e-868b-79021f41e5e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_swx_SignificantAccountingPoliciesLineItems_019e2cc5-d712-7ebb-aaa0-735d7eae7312" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue_019e2cc5-d712-7504-859e-bf163b7c65c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue_019e2cc5-d712-7504-859e-bf163b7c65c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeOther_019e2cc5-d712-7fd9-9338-9ea2d3ae6564" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_InterestIncomeOther_019e2cc5-d712-7fd9-9338-9ea2d3ae6564" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e2cc5-d712-7466-b751-52d5eb2164e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019e2cc5-d712-7466-b751-52d5eb2164e3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d712-7c29-b62b-5ad6da372e44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity_019e2cc5-d712-7c29-b62b-5ad6da372e44" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit_019e2cc5-d712-7d8b-abc2-d3b523323d67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_DefinedBenefitPlanActuarialGainLossImmediateRecognitionAsComponentInNetPeriodicBenefitCostCredit_019e2cc5-d712-7d8b-abc2-d3b523323d67" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_MiscellaneousIncomeExpense_019e2cc5-d712-7277-93dd-4e22c1e3310d" xlink:href="swx-20260630.xsd#swx_MiscellaneousIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_swx_MiscellaneousIncomeExpense_019e2cc5-d712-7277-93dd-4e22c1e3310d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e2cc5-d712-75a2-8991-20732e27bf7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherNonoperatingIncomeExpenseAbstract_019e2cc5-d712-7264-bb31-fd9ee31844b2" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e2cc5-d712-75a2-8991-20732e27bf7b" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/RevenuesScheduleofRevenueDisaggregatedbyServiceTypeandContractTypeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#RevenuesScheduleofRevenueDisaggregatedbyServiceTypeandContractTypeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/RevenuesScheduleofRevenueDisaggregatedbyServiceTypeandContractTypeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019e2cc5-d7ea-7d07-a59a-b0053e762b2a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019e2cc5-d7ea-7d07-a59a-b0053e762b2a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019e2cc5-d7ea-7284-8557-995a9a9f07f4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e2cc5-d7ea-7d07-a59a-b0053e762b2a" xlink:to="loc_srt_MajorCustomersAxis_019e2cc5-d7ea-7284-8557-995a9a9f07f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-7284-8557-995a9a9f07f4_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019e2cc5-d7ea-7284-8557-995a9a9f07f4" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-7284-8557-995a9a9f07f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019e2cc5-d7ea-7284-8557-995a9a9f07f4" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_ResidentialMember_019e2cc5-d7ea-7021-ac97-47d7d86d950b" xlink:href="swx-20260630.xsd#swx_ResidentialMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:to="loc_swx_ResidentialMember_019e2cc5-d7ea-7021-ac97-47d7d86d950b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SmallCommercialMember_019e2cc5-d7ea-7cc0-89fc-9e317c179cfb" xlink:href="swx-20260630.xsd#swx_SmallCommercialMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:to="loc_swx_SmallCommercialMember_019e2cc5-d7ea-7cc0-89fc-9e317c179cfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_LargeCommercialMember_019e2cc5-d7ea-7a45-886f-69d63b6f6d91" xlink:href="swx-20260630.xsd#swx_LargeCommercialMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:to="loc_swx_LargeCommercialMember_019e2cc5-d7ea-7a45-886f-69d63b6f6d91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IndustrialOrOtherMember_019e2cc5-d7ea-70db-901f-ffbc15844b8e" xlink:href="swx-20260630.xsd#swx_IndustrialOrOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:to="loc_swx_IndustrialOrOtherMember_019e2cc5-d7ea-70db-901f-ffbc15844b8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_TransportationMember_019e2cc5-d7ea-7cb4-a0d7-d676155442b1" xlink:href="swx-20260630.xsd#swx_TransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e2cc5-d7ea-777b-b896-04bc5fa46124" xlink:to="loc_swx_TransportationMember_019e2cc5-d7ea-7cb4-a0d7-d676155442b1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7ea-7107-94e1-8c72911eb2ef" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019e2cc5-d7ea-7d07-a59a-b0053e762b2a" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7ea-7107-94e1-8c72911eb2ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7ea-7107-94e1-8c72911eb2ef_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7ea-7107-94e1-8c72911eb2ef" xlink:to="loc_dei_EntityDomain_019e2cc5-d7ea-7107-94e1-8c72911eb2ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7ea-7b22-ab21-958eb4df505c" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7ea-7107-94e1-8c72911eb2ef" xlink:to="loc_dei_EntityDomain_019e2cc5-d7ea-7b22-ab21-958eb4df505c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7ea-7731-a4d5-f1a9540f47a9" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7ea-7b22-ab21-958eb4df505c" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7ea-7731-a4d5-f1a9540f47a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e2cc5-d7ea-772d-a16f-981c6206e7ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e2cc5-d7ea-772d-a16f-981c6206e7ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_AlternativeRevenueProgramRevenues_019e2cc5-d7ea-71a9-9d60-374158bcfb45" xlink:href="swx-20260630.xsd#swx_AlternativeRevenueProgramRevenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:to="loc_swx_AlternativeRevenueProgramRevenues_019e2cc5-d7ea-71a9-9d60-374158bcfb45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_OtherRevenue_019e2cc5-d7ea-7b40-ae85-87acb90f8981" xlink:href="swx-20260630.xsd#swx_OtherRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:to="loc_swx_OtherRevenue_019e2cc5-d7ea-7b40-ae85-87acb90f8981" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatedOperatingRevenueGas_019e2cc5-d7ea-72e8-819c-f5377e7e4cb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatedOperatingRevenueGas"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019e2cc5-d7ea-7794-ad84-e5b7235b52cd" xlink:to="loc_us-gaap_RegulatedOperatingRevenueGas_019e2cc5-d7ea-72e8-819c-f5377e7e4cb7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/RevenuesNarrativeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#RevenuesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/RevenuesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e2cc5-d7ea-7a29-8c40-657ad9d5f05e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e2cc5-d7ea-7a29-8c40-657ad9d5f05e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e2cc5-d7ea-7cf4-9392-ed47db2b6b86" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e2cc5-d7ea-7a29-8c40-657ad9d5f05e" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e2cc5-d7ea-7cf4-9392-ed47db2b6b86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e2cc5-d7ea-70f1-a35d-0e0f4506c6a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e2cc5-d7ea-70f1-a35d-0e0f4506c6a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e2cc5-d7ea-7cf6-83a1-5abc2bcba16a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e2cc5-d7ea-7cf6-83a1-5abc2bcba16a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/RevenuesNarrativeDetails_1" xlink:type="simple" xlink:href="swx-20260630.xsd#RevenuesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/RevenuesNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e2cc5-d7ea-7a29-8c40-657ad9d5f05e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e2cc5-d7ea-7a29-8c40-657ad9d5f05e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e2cc5-d7ea-70f1-a35d-0e0f4506c6a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e2cc5-d7ea-70f1-a35d-0e0f4506c6a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e2cc5-d7ea-7cf6-83a1-5abc2bcba16a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e2cc5-d7ea-7664-b58d-7d01ec972fcc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e2cc5-d7ea-7cf6-83a1-5abc2bcba16a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/DebtScheduleofCarryingAmountsandEstimatedFairValuesofLongTermDebtDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#DebtScheduleofCarryingAmountsandEstimatedFairValuesofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/DebtScheduleofCarryingAmountsandEstimatedFairValuesofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-759f-854f-4da36df6d1f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-759f-854f-4da36df6d1f1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019e2cc5-d7eb-75d5-a190-50d4f7e15936" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-759f-854f-4da36df6d1f1" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019e2cc5-d7eb-75d5-a190-50d4f7e15936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-75d5-a190-50d4f7e15936_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e2cc5-d7eb-75d5-a190-50d4f7e15936" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-75d5-a190-50d4f7e15936_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e2cc5-d7eb-707d-a566-64ce1f7b06cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019e2cc5-d7eb-75d5-a190-50d4f7e15936" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e2cc5-d7eb-707d-a566-64ce1f7b06cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-7679-bfff-bc93ee1e7643" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019e2cc5-d7eb-707d-a566-64ce1f7b06cd" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-7679-bfff-bc93ee1e7643" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e2cc5-d7eb-70bf-8755-dd2264b1277f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-7679-bfff-bc93ee1e7643" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019e2cc5-d7eb-70bf-8755-dd2264b1277f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e2cc5-d7eb-77ce-94df-ef784155ea2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019e2cc5-d7eb-7679-bfff-bc93ee1e7643" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019e2cc5-d7eb-77ce-94df-ef784155ea2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-782f-89f1-16edc58c7f56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-759f-854f-4da36df6d1f1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-782f-89f1-16edc58c7f56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-782f-89f1-16edc58c7f56_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-782f-89f1-16edc58c7f56" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-782f-89f1-16edc58c7f56_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-738d-bbe7-08d76dc4cf88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-782f-89f1-16edc58c7f56" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-738d-bbe7-08d76dc4cf88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesMember_019e2cc5-d7eb-7e4c-bab2-a5ae24833354" xlink:href="swx-20260630.xsd#swx_DebenturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-738d-bbe7-08d76dc4cf88" xlink:to="loc_swx_DebenturesMember_019e2cc5-d7eb-7e4c-bab2-a5ae24833354" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_RevolvingCreditFacilityAndCommercialPaperMember_019f6388-2eeb-76df-9037-c692afa3d47a" xlink:href="swx-20260630.xsd#swx_RevolvingCreditFacilityAndCommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-738d-bbe7-08d76dc4cf88" xlink:to="loc_swx_RevolvingCreditFacilityAndCommercialPaperMember_019f6388-2eeb-76df-9037-c692afa3d47a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IndustrialDevelopmentRevenueBondsMember_019e2cc5-d7eb-7aa7-b29c-a5fc212d27bd" xlink:href="swx-20260630.xsd#swx_IndustrialDevelopmentRevenueBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-738d-bbe7-08d76dc4cf88" xlink:to="loc_swx_IndustrialDevelopmentRevenueBondsMember_019e2cc5-d7eb-7aa7-b29c-a5fc212d27bd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-7219-87f1-4d2faa6bc924" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-759f-854f-4da36df6d1f1" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-7219-87f1-4d2faa6bc924" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7219-87f1-4d2faa6bc924_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-7219-87f1-4d2faa6bc924" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7219-87f1-4d2faa6bc924_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-7219-87f1-4d2faa6bc924" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_Debentures8SeriesDueAugust2026Member_019e2cc5-d7eb-7e05-8d13-53e0b7daad25" xlink:href="swx-20260630.xsd#swx_Debentures8SeriesDueAugust2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_Debentures8SeriesDueAugust2026Member_019e2cc5-d7eb-7e05-8d13-53e0b7daad25" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member_019e2cc5-d7eb-773b-b773-89f3096735be" xlink:href="swx-20260630.xsd#swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member_019e2cc5-d7eb-773b-b773-89f3096735be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesMediumTermNotes6.76SeriesDueSeptember2027Member_019e2cc5-d7eb-70de-ab4a-242216c56e8a" xlink:href="swx-20260630.xsd#swx_DebenturesMediumTermNotes6.76SeriesDueSeptember2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesMediumTermNotes6.76SeriesDueSeptember2027Member_019e2cc5-d7eb-70de-ab4a-242216c56e8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes5.8DueDecember2027Member_019e2cc5-d7eb-7d39-9cd2-0860fae00b3b" xlink:href="swx-20260630.xsd#swx_DebenturesNotes5.8DueDecember2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes5.8DueDecember2027Member_019e2cc5-d7eb-7d39-9cd2-0860fae00b3b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes3.7DueApril2028Member_019e2cc5-d7eb-7844-a085-77436d09de87" xlink:href="swx-20260630.xsd#swx_DebenturesNotes3.7DueApril2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes3.7DueApril2028Member_019e2cc5-d7eb-7844-a085-77436d09de87" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes5.45DueFebruary2028Member_019e2cc5-d7eb-7fea-9773-673e6abe32ee" xlink:href="swx-20260630.xsd#swx_DebenturesNotes5.45DueFebruary2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes5.45DueFebruary2028Member_019e2cc5-d7eb-7fea-9773-673e6abe32ee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes2.2DueJune2030Member_019e2cc5-d7eb-75e2-abf3-7df33ea41a16" xlink:href="swx-20260630.xsd#swx_DebenturesNotes2.2DueJune2030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes2.2DueJune2030Member_019e2cc5-d7eb-75e2-abf3-7df33ea41a16" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes4.05DueMarch2032Member_019e2cc5-d7eb-7b02-a021-8d0ec8edd1d8" xlink:href="swx-20260630.xsd#swx_DebenturesNotes4.05DueMarch2032Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes4.05DueMarch2032Member_019e2cc5-d7eb-7b02-a021-8d0ec8edd1d8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes6.1DueFebruary2041Member_019e2cc5-d7eb-7e0f-955c-3ef411ead6c3" xlink:href="swx-20260630.xsd#swx_DebenturesNotes6.1DueFebruary2041Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes6.1DueFebruary2041Member_019e2cc5-d7eb-7e0f-955c-3ef411ead6c3" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes4.875DueOctober2043Member_019e2cc5-d7eb-7708-afbc-120def866f12" xlink:href="swx-20260630.xsd#swx_DebenturesNotes4.875DueOctober2043Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes4.875DueOctober2043Member_019e2cc5-d7eb-7708-afbc-120def866f12" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes3.8DueSeptember2046Member_019e2cc5-d7eb-75f9-aa32-bb93e72d0a76" xlink:href="swx-20260630.xsd#swx_DebenturesNotes3.8DueSeptember2046Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes3.8DueSeptember2046Member_019e2cc5-d7eb-75f9-aa32-bb93e72d0a76" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes4.15DueJune2049Member_019e2cc5-d7eb-7543-b102-1af609efcbd2" xlink:href="swx-20260630.xsd#swx_DebenturesNotes4.15DueJune2049Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes4.15DueJune2049Member_019e2cc5-d7eb-7543-b102-1af609efcbd2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesNotes3.18DueAugust2051Member_019e2cc5-d7eb-77a3-b72f-919c8a2c11ba" xlink:href="swx-20260630.xsd#swx_DebenturesNotes3.18DueAugust2051Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_DebenturesNotes3.18DueAugust2051Member_019e2cc5-d7eb-77a3-b72f-919c8a2c11ba" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_VariableRateBondsTaxExemptSeriesADueDecember2028Member_019e2cc5-d7eb-781c-962d-c2bce129f1c5" xlink:href="swx-20260630.xsd#swx_VariableRateBondsTaxExemptSeriesADueDecember2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_VariableRateBondsTaxExemptSeriesADueDecember2028Member_019e2cc5-d7eb-781c-962d-c2bce129f1c5" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_VariableRateBonds2003SeriesADueMarch2038Member_019e2cc5-d7eb-7ba1-9365-d9a20cb4dbc9" xlink:href="swx-20260630.xsd#swx_VariableRateBonds2003SeriesADueMarch2038Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_VariableRateBonds2003SeriesADueMarch2038Member_019e2cc5-d7eb-7ba1-9365-d9a20cb4dbc9" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_VariableRateBonds2008SeriesADueMarch2038Member_019e2cc5-d7eb-74ed-a358-37553ea34daa" xlink:href="swx-20260630.xsd#swx_VariableRateBonds2008SeriesADueMarch2038Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_VariableRateBonds2008SeriesADueMarch2038Member_019e2cc5-d7eb-74ed-a358-37553ea34daa" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_VariableRateBonds2009SeriesADueDecember2039Member_019e2cc5-d7eb-7551-aa15-9142388c2f60" xlink:href="swx-20260630.xsd#swx_VariableRateBonds2009SeriesADueDecember2039Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7b54-b3f0-d6b2ad0fa6d7" xlink:to="loc_swx_VariableRateBonds2009SeriesADueDecember2039Member_019e2cc5-d7eb-7551-aa15-9142388c2f60" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e2cc5-d7eb-7f4c-8554-2a3f26cdb546" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e2cc5-d7eb-7f4c-8554-2a3f26cdb546" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e2cc5-d7eb-7555-9edc-09a3d72a004a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e2cc5-d7eb-7555-9edc-09a3d72a004a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019e2cc5-d7eb-780a-9b8a-e18cf0b03050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019e2cc5-d7eb-780a-9b8a-e18cf0b03050" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e2cc5-d7eb-70b4-8f67-d3328bbc8a79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_LongTermDebt_019e2cc5-d7eb-70b4-8f67-d3328bbc8a79" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019e2cc5-d7eb-7414-ba90-4eac882f935b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_LongTermDebtCurrent_019e2cc5-d7eb-7414-ba90-4eac882f935b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019e2cc5-d7eb-7cc7-ae01-17709ef6b6ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-75c9-bebc-b6d84fb51acb" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019e2cc5-d7eb-7cc7-ae01-17709ef6b6ae" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/DebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_CreditFacilityAxis_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78" xlink:to="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7ea8-9238-5de859807eae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019e2cc5-d7eb-7de6-a1f0-8f7a438bcb78" xlink:to="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7ea8-9238-5de859807eae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019e2cc5-d7eb-77b6-8c7d-e2dbffbb2140" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7ea8-9238-5de859807eae" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019e2cc5-d7eb-77b6-8c7d-e2dbffbb2140" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BridgeLoanMember_019e2cc5-d7eb-717f-8a77-d344cd9e46a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BridgeLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019e2cc5-d7eb-7ea8-9238-5de859807eae" xlink:to="loc_us-gaap_BridgeLoanMember_019e2cc5-d7eb-717f-8a77-d344cd9e46a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-76c0-a826-f947b0af8dbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-76c0-a826-f947b0af8dbc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-76c0-a826-f947b0af8dbc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-76c0-a826-f947b0af8dbc" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-76c0-a826-f947b0af8dbc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e2cc5-d7eb-76c0-a826-f947b0af8dbc" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_RevolvingCreditAgreementDueAugust2029Member_019e2cc5-d7eb-7c0c-9b75-5b8179883bb8" xlink:href="swx-20260630.xsd#swx_RevolvingCreditAgreementDueAugust2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:to="loc_swx_RevolvingCreditAgreementDueAugust2029Member_019e2cc5-d7eb-7c0c-9b75-5b8179883bb8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_CommercialPaperProgramMember_019f839c-98a1-7451-8fce-4724c21e27a6" xlink:href="swx-20260630.xsd#swx_CommercialPaperProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:to="loc_swx_CommercialPaperProgramMember_019f839c-98a1-7451-8fce-4724c21e27a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_Debentures8SeriesDueAugust2026Member_019e2cc5-d7eb-76b5-8815-507bccf54d3f" xlink:href="swx-20260630.xsd#swx_Debentures8SeriesDueAugust2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:to="loc_swx_Debentures8SeriesDueAugust2026Member_019e2cc5-d7eb-76b5-8815-507bccf54d3f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member_019f8393-42b0-7613-9ac3-bbb6dd253c99" xlink:href="swx-20260630.xsd#swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e2cc5-d7eb-7e28-ab77-64c7c0ab5357" xlink:to="loc_swx_DebenturesMediumTermNotes7.92SeriesDueJune2027Member_019f8393-42b0-7613-9ac3-bbb6dd253c99" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7f8c-b351-78b858b51163" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7f8c-b351-78b858b51163" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7f8c-b351-78b858b51163_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7f8c-b351-78b858b51163" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7f8c-b351-78b858b51163_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7b39-8389-111981e7ee59" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7f8c-b351-78b858b51163" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7b39-8389-111981e7ee59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-7cf4-a109-4fafb6b25647" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7eb-7b39-8389-111981e7ee59" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-7cf4-a109-4fafb6b25647" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-752a-89e5-da1d19d9ea77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-752a-89e5-da1d19d9ea77" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-752a-89e5-da1d19d9ea77_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-752a-89e5-da1d19d9ea77" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-752a-89e5-da1d19d9ea77_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-7856-8377-30cc925e0174" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e2cc5-d7eb-752a-89e5-da1d19d9ea77" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-7856-8377-30cc925e0174" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e2cc5-d7eb-78f4-8fd0-d78be5198762" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-7856-8377-30cc925e0174" xlink:to="loc_us-gaap_LineOfCreditMember_019e2cc5-d7eb-78f4-8fd0-d78be5198762" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebenturesMember_019e2cc5-d7eb-7d64-b934-90370d7fcc90" xlink:href="swx-20260630.xsd#swx_DebenturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e2cc5-d7eb-7856-8377-30cc925e0174" xlink:to="loc_swx_DebenturesMember_019e2cc5-d7eb-7d64-b934-90370d7fcc90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeAxis_019e2cc5-d7eb-7e5d-9b8f-db27723b1119" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_ShortTermDebtTypeAxis_019e2cc5-d7eb-7e5d-9b8f-db27723b1119" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7e5d-9b8f-db27723b1119_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019e2cc5-d7eb-7e5d-9b8f-db27723b1119" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7e5d-9b8f-db27723b1119_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7fb7-b720-6856bdb3bc2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShortTermDebtTypeAxis_019e2cc5-d7eb-7e5d-9b8f-db27723b1119" xlink:to="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7fb7-b720-6856bdb3bc2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019e2cc5-d7eb-7464-9828-7045458c3d73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7fb7-b720-6856bdb3bc2f" xlink:to="loc_us-gaap_LineOfCreditMember_019e2cc5-d7eb-7464-9828-7045458c3d73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019f839c-98a1-7cd2-9793-b510fa555a02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermDebtTypeDomain_019e2cc5-d7eb-7fb7-b720-6856bdb3bc2f" xlink:to="loc_us-gaap_CommercialPaperMember_019f839c-98a1-7cd2-9793-b510fa555a02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019e2cc5-d7eb-71d4-88c5-7ae6315819c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_VariableRateAxis_019e2cc5-d7eb-71d4-88c5-7ae6315819c5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-71d4-88c5-7ae6315819c5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019e2cc5-d7eb-71d4-88c5-7ae6315819c5" xlink:to="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-71d4-88c5-7ae6315819c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-7c55-bc72-66777c8e15b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019e2cc5-d7eb-71d4-88c5-7ae6315819c5" xlink:to="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-7c55-bc72-66777c8e15b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e2cc5-d7eb-7a61-b86e-fd54fe081fc0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-7c55-bc72-66777c8e15b3" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019e2cc5-d7eb-7a61-b86e-fd54fe081fc0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019e2cc5-d7eb-732a-a3dc-8e9172b280d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019e2cc5-d7eb-7c55-bc72-66777c8e15b3" xlink:to="loc_us-gaap_BaseRateMember_019e2cc5-d7eb-732a-a3dc-8e9172b280d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e2cc5-d7eb-7f94-9200-70caf4cde1fb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_srt_RangeAxis_019e2cc5-d7eb-7f94-9200-70caf4cde1fb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e2cc5-d7eb-7f94-9200-70caf4cde1fb_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e2cc5-d7eb-7f94-9200-70caf4cde1fb" xlink:to="loc_srt_RangeMember_019e2cc5-d7eb-7f94-9200-70caf4cde1fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e2cc5-d7eb-71a0-a503-33aae2e02cc0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e2cc5-d7eb-7f94-9200-70caf4cde1fb" xlink:to="loc_srt_RangeMember_019e2cc5-d7eb-71a0-a503-33aae2e02cc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e2cc5-d7eb-7a0b-8155-3a349a1182b4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e2cc5-d7eb-71a0-a503-33aae2e02cc0" xlink:to="loc_srt_MinimumMember_019e2cc5-d7eb-7a0b-8155-3a349a1182b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e2cc5-d7eb-7d85-a943-426f051a2668" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e2cc5-d7eb-71a0-a503-33aae2e02cc0" xlink:to="loc_srt_MaximumMember_019e2cc5-d7eb-7d85-a943-426f051a2668" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019f839e-7e12-7dd1-9f46-b0ff9c3c84af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e2cc5-d7eb-7c2e-b0f6-e41cc3485388" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019f839e-7e12-7dd1-9f46-b0ff9c3c84af" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f839e-7e12-7dd1-9f46-b0ff9c3c84af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f839e-7e12-7dd1-9f46-b0ff9c3c84af" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f839e-7e12-7dd1-9f46-b0ff9c3c84af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019f839e-7e12-7d19-95f6-ba3f92db8453" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019f839e-7e12-7dd1-9f46-b0ff9c3c84af" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019f839e-7e12-7d19-95f6-ba3f92db8453" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019f839e-a0ea-7d49-9a1b-65ea26aa882d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019f839e-7e12-7d19-95f6-ba3f92db8453" xlink:to="loc_us-gaap_SubsequentEventMember_019f839e-a0ea-7d49-9a1b-65ea26aa882d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e2cc5-d7eb-7875-9cec-c9459e259f4c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019e2cc5-d7eb-7875-9cec-c9459e259f4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebtInstrumentInterestPeriod_019e2cc5-d7eb-748f-92b5-2f9c610c04b8" xlink:href="swx-20260630.xsd#swx_DebtInstrumentInterestPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_swx_DebtInstrumentInterestPeriod_019e2cc5-d7eb-748f-92b5-2f9c610c04b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_DebtInstrumentBasisSpreadOnInterestVariableRate_019e2cc5-d7eb-7015-946a-b74017659a5f" xlink:href="swx-20260630.xsd#swx_DebtInstrumentBasisSpreadOnInterestVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_swx_DebtInstrumentBasisSpreadOnInterestVariableRate_019e2cc5-d7eb-7015-946a-b74017659a5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e2cc5-d7eb-764f-9806-1105cbda9cb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019e2cc5-d7eb-764f-9806-1105cbda9cb7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e2cc5-d7eb-7e8f-ab35-477df619b6e6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019e2cc5-d7eb-7e8f-ab35-477df619b6e6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e2cc5-d7eb-7c2a-8360-5f0745becc5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_LongTermDebt_019e2cc5-d7eb-7c2a-8360-5f0745becc5d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_CommercialPaperMaximumIssuanceCapacity_019f839c-98a1-7c31-ad88-3d466ff6e4b5" xlink:href="swx-20260630.xsd#swx_CommercialPaperMaximumIssuanceCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_swx_CommercialPaperMaximumIssuanceCapacity_019f839c-98a1-7c31-ad88-3d466ff6e4b5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e2cc5-d7eb-7e17-af41-7638afa12eed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e2cc5-d7eb-7e17-af41-7638afa12eed" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e2cc5-d7eb-7b87-b0d9-ae7af95f4a4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e2cc5-d7eb-7b87-b0d9-ae7af95f4a4a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermDebtWeightedAverageInterestRate_019e2cc5-d7eb-786e-893f-3c3309eecccb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShortTermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e2cc5-d7eb-7964-837e-bed409ec5097" xlink:to="loc_us-gaap_ShortTermDebtWeightedAverageInterestRate_019e2cc5-d7eb-786e-893f-3c3309eecccb" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/ComponentsofNetPeriodicBenefitCostDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#ComponentsofNetPeriodicBenefitCostDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/ComponentsofNetPeriodicBenefitCostDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e2cc5-d7eb-7db9-9992-ac64d7b7fbce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e2cc5-d7eb-7db9-9992-ac64d7b7fbce" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeAxis_019e2cc5-d7eb-773a-9878-488d5a26fd34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_019e2cc5-d7eb-7db9-9992-ac64d7b7fbce" xlink:to="loc_us-gaap_RetirementPlanTypeAxis_019e2cc5-d7eb-773a-9878-488d5a26fd34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-773a-9878-488d5a26fd34_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019e2cc5-d7eb-773a-9878-488d5a26fd34" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-773a-9878-488d5a26fd34_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-7f52-baa2-0094c8208612" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetirementPlanTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RetirementPlanTypeAxis_019e2cc5-d7eb-773a-9878-488d5a26fd34" xlink:to="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-7f52-baa2-0094c8208612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPensionPlansDefinedBenefitMember_019e2cc5-d7eb-7204-a0d0-af9676c8879b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherPensionPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-7f52-baa2-0094c8208612" xlink:to="loc_us-gaap_OtherPensionPlansDefinedBenefitMember_019e2cc5-d7eb-7204-a0d0-af9676c8879b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_019e2cc5-d7eb-7aa5-9733-a94dc0a36fd8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-7f52-baa2-0094c8208612" xlink:to="loc_us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember_019e2cc5-d7eb-7aa5-9733-a94dc0a36fd8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_019e2cc5-d7eb-78f5-a0da-44215637b7db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RetirementPlanTypeDomain_019e2cc5-d7eb-7f52-baa2-0094c8208612" xlink:to="loc_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember_019e2cc5-d7eb-78f5-a0da-44215637b7db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanServiceCost_019e2cc5-d7eb-7413-b3df-8868758ededc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanServiceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanServiceCost_019e2cc5-d7eb-7413-b3df-8868758ededc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanInterestCost_019e2cc5-d7eb-73dc-b5aa-b92689561f74" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanInterestCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanInterestCost_019e2cc5-d7eb-73dc-b5aa-b92689561f74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_019e2cc5-d7eb-71cc-8b3e-3ce28d3c52da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets_019e2cc5-d7eb-71cc-8b3e-3ce28d3c52da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_019e2cc5-d7eb-747c-9eb1-76a3de1a50eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit_019e2cc5-d7eb-747c-9eb1-76a3de1a50eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_019e2cc5-d7eb-79b2-b3bd-09e44fb2faaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses_019e2cc5-d7eb-79b2-b3bd-09e44fb2faaa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_019e2cc5-d7eb-7562-8333-dbebac94a99d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_019e2cc5-d7eb-73ff-aaf8-9f609873d828" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_019e2cc5-d7eb-7562-8333-dbebac94a99d" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsNarrativeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#PreviouslyIssuedCondensedConsolidatedFinancialStatementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019f1e08-92cd-75c0-aaff-f03c6e117e71" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019f1e08-92cd-7791-b0ff-4f6576265e77" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019f1e08-92cd-75c0-aaff-f03c6e117e71" xlink:to="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019f1e08-92cd-7791-b0ff-4f6576265e77" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1e08-92cd-79ec-808b-67c1071a492b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019f1e08-92cd-7791-b0ff-4f6576265e77" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1e08-92cd-79ec-808b-67c1071a492b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1e08-92cd-79ec-808b-67c1071a492b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1e08-92cd-79ec-808b-67c1071a492b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1e08-92cd-79ec-808b-67c1071a492b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1e08-92cd-70a1-b4c7-17726a95e54f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019f1e08-92cd-79ec-808b-67c1071a492b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1e08-92cd-70a1-b4c7-17726a95e54f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_PublicOfferingAndPrivatePlacementMember_019f1e08-92cd-7c13-979f-fabe27a244c5" xlink:href="swx-20260630.xsd#swx_PublicOfferingAndPrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019f1e08-92cd-70a1-b4c7-17726a95e54f" xlink:to="loc_swx_PublicOfferingAndPrivatePlacementMember_019f1e08-92cd-7c13-979f-fabe27a244c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019f1e08-92cd-7c4f-8a72-f1eeeca845f1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019f1e08-92cd-7791-b0ff-4f6576265e77" xlink:to="loc_srt_RestatementAxis_019f1e08-92cd-7c4f-8a72-f1eeeca845f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019f1e08-92cd-7c4f-8a72-f1eeeca845f1_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019f1e08-92cd-7c4f-8a72-f1eeeca845f1" xlink:to="loc_srt_RestatementDomain_019f1e08-92cd-7c4f-8a72-f1eeeca845f1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019f1e08-92cd-74b4-bb39-a0568cf69722" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019f1e08-92cd-7c4f-8a72-f1eeeca845f1" xlink:to="loc_srt_RestatementDomain_019f1e08-92cd-74b4-bb39-a0568cf69722" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019f1e08-92cd-7240-94ff-9024b002d046" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019f1e08-92cd-74b4-bb39-a0568cf69722" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019f1e08-92cd-7240-94ff-9024b002d046" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1e08-92cd-71a6-af0e-30b40c79fadf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019f1e08-92cd-75c0-aaff-f03c6e117e71" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019f1e08-92cd-71a6-af0e-30b40c79fadf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019f1e08-92cd-7427-b9eb-204bb506185d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019f1e08-92cd-75c0-aaff-f03c6e117e71" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019f1e08-92cd-7427-b9eb-204bb506185d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilities_019f1e08-92cd-7a55-9379-f83569e99e8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019f1e08-92cd-75c0-aaff-f03c6e117e71" xlink:to="loc_us-gaap_DeferredTaxLiabilities_019f1e08-92cd-7a55-9379-f83569e99e8e" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofIncomeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7fda-b252-58767ab0ae83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7fda-b252-58767ab0ae83" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7fda-b252-58767ab0ae83" xlink:to="loc_srt_RestatementAxis_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-7ed5-a34a-4fa453103c7d" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7af6-b0d9-79c47adfdaf3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7af6-b0d9-79c47adfdaf3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadaf-119c-77d5-a5ae-8e130c9720bf" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadaf-119c-77d5-a5ae-8e130c9720bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_RevisionOfPriorPeriodAsRestatedMember_019f1e0f-a50d-7376-848d-6358ee3b0042" xlink:href="swx-20260630.xsd#swx_RevisionOfPriorPeriodAsRestatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:to="loc_swx_RevisionOfPriorPeriodAsRestatedMember_019f1e0f-a50d-7376-848d-6358ee3b0042" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_019fada0-0aa4-7bf0-8087-0629e21648dd" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodReclassificationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-76fd-a734-4bce00f45e7e" xlink:to="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_019fada0-0aa4-7bf0-8087-0629e21648dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7eb-728f-8a72-e813ff9e4663" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7fda-b252-58767ab0ae83" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7eb-728f-8a72-e813ff9e4663" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-728f-8a72-e813ff9e4663_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-728f-8a72-e813ff9e4663" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-728f-8a72-e813ff9e4663_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-765c-800b-6ef2adc35ae7" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-728f-8a72-e813ff9e4663" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-765c-800b-6ef2adc35ae7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-707c-aa21-8fa81d038293" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7eb-765c-800b-6ef2adc35ae7" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-707c-aa21-8fa81d038293" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_RegulatedOperatingRevenueUtilityInfrastructureServices_019f1e0f-a50d-7bad-998e-3b091c3e35f9" xlink:href="swx-20260630.xsd#swx_RegulatedOperatingRevenueUtilityInfrastructureServices"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_swx_RegulatedOperatingRevenueUtilityInfrastructureServices_019f1e0f-a50d-7bad-998e-3b091c3e35f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RegulatedAndUnregulatedOperatingRevenue_019e2cc5-d7eb-7775-8d8c-b54322afbb2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RegulatedAndUnregulatedOperatingRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_RegulatedAndUnregulatedOperatingRevenue_019e2cc5-d7eb-7775-8d8c-b54322afbb2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_019f1e0f-a50d-700f-b451-c05cf2f40346" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations_019f1e0f-a50d-700f-b451-c05cf2f40346" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019f1e0f-a50d-7e88-8e82-136b130b8e43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_DepreciationAndAmortization_019f1e0f-a50d-7e88-8e82-136b130b8e43" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019f1e0f-a50d-792d-a2bf-907b6b518eae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019f1e0f-a50d-792d-a2bf-907b6b518eae" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostsAndExpenses_019f1e0f-a50d-7be0-89c6-6b83602585fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_CostsAndExpenses_019f1e0f-a50d-7be0-89c6-6b83602585fa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019e2cc5-d7eb-760c-bc39-ce144e8855a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_OperatingIncomeLoss_019e2cc5-d7eb-760c-bc39-ce144e8855a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019f1e0f-a50d-763e-a7f2-8e71b9f91b4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019f1e0f-a50d-763e-a7f2-8e71b9f91b4b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019f1e0f-a50d-756f-844e-3374e64ed56f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019f1e0f-a50d-756f-844e-3374e64ed56f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_019f1e0f-a50d-7621-a3a5-6dc107592159" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_019f1e0f-a50d-7621-a3a5-6dc107592159" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e2cc5-d7eb-7bbc-86ce-5bbd49cd77b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019e2cc5-d7eb-7bbc-86ce-5bbd49cd77b4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e2cc5-d7eb-7091-bb7b-1f9de2966f00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e2cc5-d7eb-7091-bb7b-1f9de2966f00" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019f1e0f-a50d-7bc3-ac2c-04aed21c1c40" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019f1e0f-a50d-7bc3-ac2c-04aed21c1c40" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019f1e0f-a50d-7c8f-99d4-c49d90654980" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019f1e0f-a50d-7c8f-99d4-c49d90654980" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019f1e0f-a50d-7542-8f80-2d4e6df06203" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_ProfitLoss_019f1e0f-a50d-7542-8f80-2d4e6df06203" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e2cc5-d7eb-7ef7-8b07-fea5a1f5f1cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_NetIncomeLoss_019e2cc5-d7eb-7ef7-8b07-fea5a1f5f1cd" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_019f1e12-f6c0-7856-a5d2-e4788a3c5a01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_019f1e12-f6c0-7856-a5d2-e4788a3c5a01" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_019f1e0f-a50d-72b2-8d3d-02b0caa9587a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_019f1e12-f6c0-7856-a5d2-e4788a3c5a01" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerBasicShare_019f1e0f-a50d-72b2-8d3d-02b0caa9587a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_019f1e0f-a50d-7093-a3ad-8d08359a94f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_019f1e12-f6c0-7856-a5d2-e4788a3c5a01" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare_019f1e0f-a50d-7093-a3ad-8d08359a94f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019f1e0f-a50d-74ba-8644-fae65a9c8777" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_019f1e12-f6c0-7856-a5d2-e4788a3c5a01" xlink:to="loc_us-gaap_EarningsPerShareBasic_019f1e0f-a50d-74ba-8644-fae65a9c8777" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_019f1e13-4cdf-7cc9-a93d-753a6403afab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_019f1e13-4cdf-7cc9-a93d-753a6403afab" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_019f1e0f-a50d-794b-9e5a-28955f34718b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_019f1e13-4cdf-7cc9-a93d-753a6403afab" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare_019f1e0f-a50d-794b-9e5a-28955f34718b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_019f1e0f-a50d-7419-98ab-9b17eff02498" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_019f1e13-4cdf-7cc9-a93d-753a6403afab" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare_019f1e0f-a50d-7419-98ab-9b17eff02498" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019f1e0f-a50d-7a2b-baaf-7b774a744480" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_019f1e13-4cdf-7cc9-a93d-753a6403afab" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019f1e0f-a50d-7a2b-baaf-7b774a744480" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f1e13-9e48-7a1e-a741-4b20edeed878" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7b90-8939-b497b7a24f97" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f1e13-9e48-7a1e-a741-4b20edeed878" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019f1e0f-a50d-751a-8f51-2f7cfad9e151" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f1e13-9e48-7a1e-a741-4b20edeed878" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019f1e0f-a50d-751a-8f51-2f7cfad9e151" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019f1e0f-a50d-717d-8f5d-c9c6159cd5be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_019f1e13-9e48-7a1e-a741-4b20edeed878" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019f1e0f-a50d-717d-8f5d-c9c6159cd5be" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofComprehensiveIncomeDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofComprehensiveIncomeDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofComprehensiveIncomeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7ea6-bd4d-160e92ece645" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-75fa-9db4-61a6cea17734" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7ea6-bd4d-160e92ece645" xlink:to="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-75fa-9db4-61a6cea17734" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019e2cc5-d7eb-7a14-ab31-417f13329331" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-75fa-9db4-61a6cea17734" xlink:to="loc_srt_RestatementAxis_019e2cc5-d7eb-7a14-ab31-417f13329331" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-7a14-ab31-417f13329331_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-7a14-ab31-417f13329331" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-7a14-ab31-417f13329331_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-7050-ad9c-b36a0c6d53d8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-7a14-ab31-417f13329331" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-7050-ad9c-b36a0c6d53d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7cce-bba0-fff076e25107" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-7050-ad9c-b36a0c6d53d8" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7cce-bba0-fff076e25107" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadb9-78fc-7a0b-a0ee-3fb63ba19372" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-7050-ad9c-b36a0c6d53d8" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadb9-78fc-7a0b-a0ee-3fb63ba19372" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7eb-77dc-894d-b85c41520e28" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-75fa-9db4-61a6cea17734" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7eb-77dc-894d-b85c41520e28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-77dc-894d-b85c41520e28_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-77dc-894d-b85c41520e28" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-77dc-894d-b85c41520e28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7ee7-91b7-970e73467335" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-77dc-894d-b85c41520e28" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7ee7-91b7-970e73467335" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-7800-9dfb-89d63b10ed12" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7eb-7ee7-91b7-970e73467335" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-7800-9dfb-89d63b10ed12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e2cc5-d7eb-7ee5-98e6-050729b405c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7ea6-bd4d-160e92ece645" xlink:to="loc_us-gaap_ProfitLoss_019e2cc5-d7eb-7ee5-98e6-050729b405c0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d7eb-7851-a56b-3f7546cc9409" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7ea6-bd4d-160e92ece645" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d7eb-7851-a56b-3f7546cc9409" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_019f1e1b-b38a-71f9-8a25-7c04fc24c79b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComprehensiveIncomeNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7ea6-bd4d-160e92ece645" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_019f1e1b-b38a-71f9-8a25-7c04fc24c79b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofCashFlowsDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofCashFlowsDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofCashFlowsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7824-a874-7261263d9df8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:to="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7824-a874-7261263d9df8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7824-a874-7261263d9df8" xlink:to="loc_srt_RestatementAxis_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-778a-8150-a2f6e5aecb3d" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7509-9915-f58fabef428c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-7509-9915-f58fabef428c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadaf-7dd4-7f57-b080-7071209c95e8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadaf-7dd4-7f57-b080-7071209c95e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_RevisionOfPriorPeriodAsRestatedMember_019f1e1d-5a4f-7f7e-945d-bd69851d64ad" xlink:href="swx-20260630.xsd#swx_RevisionOfPriorPeriodAsRestatedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:to="loc_swx_RevisionOfPriorPeriodAsRestatedMember_019f1e1d-5a4f-7f7e-945d-bd69851d64ad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_019fadb0-42fe-757d-b1e3-697edf91ace8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodReclassificationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-73bf-8382-2fb46a69ea69" xlink:to="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_019fadb0-42fe-757d-b1e3-697edf91ace8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7824-a874-7261263d9df8" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7e62-9334-6a458b515be3" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-7e64-bbde-bd8afd7876ef" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7e62-9334-6a458b515be3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-71ac-9c24-58ebfe655fe1" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7eb-7e62-9334-6a458b515be3" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-71ac-9c24-58ebfe655fe1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOperatingActivitiesSegmentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-7824-a874-7261263d9df8" xlink:to="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7cf1-9ac3-046e64400c9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentOperatingActivitiesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementOperatingActivitiesSegmentAxis_019f1e1d-5a4f-7f6b-b72b-20ba3e65177d" xlink:to="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7cf1-9ac3-046e64400c9a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentContinuingOperationsMember_019f1e1d-5a4f-7b7c-a9af-61b399c8ac85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentContinuingOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7cf1-9ac3-046e64400c9a" xlink:to="loc_us-gaap_SegmentContinuingOperationsMember_019f1e1d-5a4f-7b7c-a9af-61b399c8ac85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDiscontinuedOperationsMember_019f1e1d-5a4f-75b0-a7e6-b8083226bdef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentOperatingActivitiesDomain_019f1e1d-5a4f-7cf1-9ac3-046e64400c9a" xlink:to="loc_us-gaap_SegmentDiscontinuedOperationsMember_019f1e1d-5a4f-75b0-a7e6-b8083226bdef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_019e2cc5-d7eb-793a-8b56-0824b35d0c8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_ProfitLoss_019e2cc5-d7eb-793a-8b56-0824b35d0c8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019f1e1d-5a4f-78e4-83e4-77569e37ee23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019f1e1d-5a4f-78e4-83e4-77569e37ee23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019f1e1d-5a4f-7c7d-8a49-dd2ca18bed44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest_019f1e1d-5a4f-7c7d-8a49-dd2ca18bed44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationAndAmortization_019f1e1d-5a4f-75b5-9e53-b2322f0716b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:to="loc_us-gaap_DepreciationAndAmortization_019f1e1d-5a4f-75b5-9e53-b2322f0716b1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d7eb-78cf-baa0-956f429e06cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_019e2cc5-d7eb-78cf-baa0-956f429e06cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019f1e1d-5a4f-74fd-905c-083d3804688c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_019f1e1d-5a4f-74fd-905c-083d3804688c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_019f1e1d-5a4f-7b8e-85b7-d0962d9ef366" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:to="loc_us-gaap_ShareBasedCompensation_019f1e1d-5a4f-7b8e-85b7-d0962d9ef366" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-770c-90f7-8adba69e1b12" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d7eb-72ed-8f4d-ebd33fee60fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_019e2cc5-d7eb-72ed-8f4d-ebd33fee60fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d7eb-73e5-a90f-a6f3c52edb6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInDeferredGasCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredGasCost_019e2cc5-d7eb-73e5-a90f-a6f3c52edb6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d7eb-7504-bebc-0e64a196d8bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_019e2cc5-d7eb-7504-bebc-0e64a196d8bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019f1e1d-5a4f-7f68-b28e-858a40636f6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInAccruedTaxesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedTaxesPayable_019f1e1d-5a4f-7f68-b28e-858a40636f6d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019f1e1d-5a4f-793c-9343-e034ecfd54dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_019e2cc5-d7eb-75c2-a928-94fe289c6a00" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherCurrentAssetsAndLiabilitiesNet_019f1e1d-5a4f-793c-9343-e034ecfd54dc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019f1e1d-5a4f-79d6-b69c-e8f31c3a6e5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_019f1e1d-5a4f-79d6-b69c-e8f31c3a6e5d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019f1e1d-5a4f-7fa4-aa65-8ff6d3e61fa8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_019f1e1d-5a4f-7fa4-aa65-8ff6d3e61fa8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations_019f1e1d-5a4f-7182-834e-049d7bfe60aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations_019f1e1d-5a4f-7182-834e-049d7bfe60aa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations_019f1e1d-5a4f-78be-87f5-68412ce1960d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations_019f1e1d-5a4f-78be-87f5-68412ce1960d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d7eb-72cb-8367-04f72594ae4a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_019e2cc5-d7eb-7a9e-bd31-c6ef0ebb75ef" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_019e2cc5-d7eb-72cb-8367-04f72594ae4a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f1e1d-5a4f-795d-9a33-31a66d2be3c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_019f1e1d-5a4f-795d-9a33-31a66d2be3c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019f1e1d-5a4f-738c-9ed0-9716dbabfabd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForProceedsFromOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:to="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_019f1e1d-5a4f-738c-9ed0-9716dbabfabd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_019f1e1d-5a4f-7798-84c0-82d1d124273e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_019f1e1d-5a4f-7798-84c0-82d1d124273e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations_019f1e1d-5a4f-71b0-b124-47c1219d9024" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:to="loc_us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations_019f1e1d-5a4f-71b0-b124-47c1219d9024" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019f1e1d-5a4f-7fc1-990d-6ae26ad12d2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_019f1e22-e910-77a5-aa02-d79bd024e6f5" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_019f1e1d-5a4f-7fc1-990d-6ae26ad12d2f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019f1e1d-5a4f-7d72-887b-55bc4c2739ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_019f1e1d-5a4f-7d72-887b-55bc4c2739ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_019f1e1d-5a4f-7a0c-8b03-15e7dd5b8895" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_019f1e1d-5a4f-7a0c-8b03-15e7dd5b8895" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019fb24d-cf80-7f2c-8ec1-2dc21d955ca2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_019fb24d-cf80-7f2c-8ec1-2dc21d955ca2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_ProceedsFromSaleOfEquityInterestInConsolidatedSubsidiariesFinancingActivity_019f1e1d-5a4f-750c-afba-f910c13d1db7" xlink:href="swx-20260630.xsd#swx_ProceedsFromSaleOfEquityInterestInConsolidatedSubsidiariesFinancingActivity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_swx_ProceedsFromSaleOfEquityInterestInConsolidatedSubsidiariesFinancingActivity_019f1e1d-5a4f-750c-afba-f910c13d1db7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019f1e1d-5a4f-71d5-a6b3-ce4ec5558fc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromPaymentsForOtherFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_019f1e1d-5a4f-71d5-a6b3-ce4ec5558fc1" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_019f1e1d-5a4f-7f0d-9032-bc3254e7a311" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_019f1e1d-5a4f-7f0d-9032-bc3254e7a311" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations_019f1e1d-5a4f-7b86-8b57-ea95e5543b82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_CashProvidedByUsedInFinancingActivitiesDiscontinuedOperations_019f1e1d-5a4f-7b86-8b57-ea95e5543b82" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019f1e1d-5a4f-79aa-933e-d4126dd77262" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_019f1e23-b5c0-7a8a-9122-060b1b61b852" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_019f1e1d-5a4f-79aa-933e-d4126dd77262" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_019f1e25-6bdd-7e80-8cf9-2f63198900f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-78d4-a47b-c0194edb3722" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_019f1e25-6bdd-7e80-8cf9-2f63198900f4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_019f1e1d-5a4f-7632-909a-9055e9e567b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019f1e25-6bdd-7e80-8cf9-2f63198900f4" xlink:to="loc_us-gaap_InterestPaidNet_019f1e1d-5a4f-7632-909a-9055e9e567b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019f1e1d-5a4f-7b0d-8d67-091f568dadbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019f1e25-6bdd-7e80-8cf9-2f63198900f4" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019f1e1d-5a4f-7b0d-8d67-091f568dadbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019f1e1d-5a4f-7b21-b3a5-9764cd7a3fd6" xlink:href="swx-20260630.xsd#swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_019f1e25-6bdd-7e80-8cf9-2f63198900f4" xlink:to="loc_swx_IncreaseDecreaseInCapitalExpendituresIncurredButNotYetPaid_019f1e1d-5a4f-7b21-b3a5-9764cd7a3fd6" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofEquityDetails" xlink:type="simple" xlink:href="swx-20260630.xsd#PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofEquityDetails"/>
  <link:definitionLink xlink:role="http://www.swgas.com/role/PreviouslyIssuedCondensedConsolidatedFinancialStatementsScheduleofEffectsofthePriorPeriodErrorsontheCondensedConsolidatedStatementsofEquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7f42-9ac3-e90cd069c257" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-74e2-a39a-22a5be66b4fe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7f42-9ac3-e90cd069c257" xlink:to="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-74e2-a39a-22a5be66b4fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_019e2cc5-d7eb-736a-a324-1254a2908ec5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-74e2-a39a-22a5be66b4fe" xlink:to="loc_srt_RestatementAxis_019e2cc5-d7eb-736a-a324-1254a2908ec5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-736a-a324-1254a2908ec5_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-736a-a324-1254a2908ec5" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-736a-a324-1254a2908ec5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_019e2cc5-d7eb-71f5-a6cf-0acedccea0b9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_019e2cc5-d7eb-736a-a324-1254a2908ec5" xlink:to="loc_srt_RestatementDomain_019e2cc5-d7eb-71f5-a6cf-0acedccea0b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-788f-a583-0769382e3468" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ScenarioPreviouslyReportedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-71f5-a6cf-0acedccea0b9" xlink:to="loc_srt_ScenarioPreviouslyReportedMember_019e2cc5-d7eb-788f-a583-0769382e3468" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadc0-25c7-7bcc-9cef-70375307158a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_019e2cc5-d7eb-71f5-a6cf-0acedccea0b9" xlink:to="loc_srt_RevisionOfPriorPeriodErrorCorrectionAdjustmentMember_019fadc0-25c7-7bcc-9cef-70375307158a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-74e2-a39a-22a5be66b4fe" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7660-a501-454f6daa8a5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e2cc5-d7eb-7e3e-a2f2-cb22c70a7223" xlink:to="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7660-a501-454f6daa8a5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e2cc5-d7eb-792c-aef0-a9a9b7d12ab9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7660-a501-454f6daa8a5e" xlink:to="loc_us-gaap_RetainedEarningsMember_019e2cc5-d7eb-792c-aef0-a9a9b7d12ab9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ParentMember_019f1dfb-fee3-79f5-9330-b6fa0d82bbb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ParentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e2cc5-d7eb-7660-a501-454f6daa8a5e" xlink:to="loc_us-gaap_ParentMember_019f1dfb-fee3-79f5-9330-b6fa0d82bbb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfErrorCorrectionsAndPriorPeriodAdjustmentRestatementTable_019e2cc5-d7eb-74e2-a39a-22a5be66b4fe" xlink:to="loc_dei_LegalEntityAxis_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019e2cc5-d7eb-7b64-a06d-3aca2095cd76" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019e2cc5-d7eb-70f3-8714-e56a9daa9fbc" xlink:to="loc_dei_EntityDomain_019e2cc5-d7eb-7b64-a06d-3aca2095cd76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-78f9-a176-476aa85a0c74" xlink:href="swx-20260630.xsd#swx_SouthwestGasCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019e2cc5-d7eb-7b64-a06d-3aca2095cd76" xlink:to="loc_swx_SouthwestGasCorporationMember_019e2cc5-d7eb-78f9-a176-476aa85a0c74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d7eb-7028-8ef9-09d6c314ce1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementLineItems_019e2cc5-d7eb-7f42-9ac3-e90cd069c257" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d7eb-7028-8ef9-09d6c314ce1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d7eb-7f13-8e60-60cc04680ea6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d7eb-7028-8ef9-09d6c314ce1e" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d7eb-7f13-8e60-60cc04680ea6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d7eb-7a3e-81a9-964289f3c1fb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e2cc5-d7eb-7028-8ef9-09d6c314ce1e" xlink:to="loc_us-gaap_NetIncomeLossIncludingPortionAttributableToNonredeemableNoncontrollingInterest_019e2cc5-d7eb-7a3e-81a9-964289f3c1fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e2cc5-d7eb-758a-8006-a0d4497d8577" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
</link:linkbase>
