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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019f2411-8062-7ad6-b69c-9909c448128f,g:8ba746ce-e9ba-459b-8f26-8b94c4fbec31-->
<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.unum.com/role/DocumentandEntityInformation" xlink:type="simple" xlink:href="unm-20260702.xsd#DocumentandEntityInformation"/>
  <link:definitionLink xlink:role="http://www.unum.com/role/DocumentandEntityInformation" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInformationLineItems_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitiesTable_019f2411-8062-7171-bf3b-46928d46647b" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntitiesTable_019f2411-8062-7171-bf3b-46928d46647b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsExchangeAxis_019f2411-8062-73b9-ae4c-ca4905b73d25" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityListingsExchangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_019f2411-8062-7171-bf3b-46928d46647b" xlink:to="loc_dei_EntityListingsExchangeAxis_019f2411-8062-73b9-ae4c-ca4905b73d25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_019f2411-8062-73b9-ae4c-ca4905b73d25_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityListingsExchangeAxis_019f2411-8062-73b9-ae4c-ca4905b73d25" xlink:to="loc_dei_ExchangeDomain_019f2411-8062-73b9-ae4c-ca4905b73d25_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_019f2411-8062-789a-a401-df2194f13cbd" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityListingsExchangeAxis_019f2411-8062-73b9-ae4c-ca4905b73d25" xlink:to="loc_dei_ExchangeDomain_019f2411-8062-789a-a401-df2194f13cbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_exch_XNYS_019f2411-8062-7844-ba33-ab35eed93537" xlink:href="https://xbrl.sec.gov/exch/2026/exch-2026.xsd#exch_XNYS"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_019f2411-8062-789a-a401-df2194f13cbd" xlink:to="loc_exch_XNYS_019f2411-8062-7844-ba33-ab35eed93537" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019f2411-8062-7afa-b9d0-6a9c3fef9d29" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_EntitiesTable_019f2411-8062-7171-bf3b-46928d46647b" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019f2411-8062-7afa-b9d0-6a9c3fef9d29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f2411-8062-7afa-b9d0-6a9c3fef9d29_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f2411-8062-7afa-b9d0-6a9c3fef9d29" xlink:to="loc_us-gaap_ClassOfStockDomain_019f2411-8062-7afa-b9d0-6a9c3fef9d29_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019f2411-8062-702b-b29d-6fdcc8b0b1e2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019f2411-8062-7afa-b9d0-6a9c3fef9d29" xlink:to="loc_us-gaap_ClassOfStockDomain_019f2411-8062-702b-b29d-6fdcc8b0b1e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019f2411-8062-739b-a9cc-1333fb81b187" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f2411-8062-702b-b29d-6fdcc8b0b1e2" xlink:to="loc_us-gaap_CommonClassAMember_019f2411-8062-739b-a9cc-1333fb81b187" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_unm_A6.250JuniorSubordinatedNotesdue2058Member_019f2411-8062-76c9-8eca-71b591cc5af5" xlink:href="unm-20260702.xsd#unm_A6.250JuniorSubordinatedNotesdue2058Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019f2411-8062-702b-b29d-6fdcc8b0b1e2" xlink:to="loc_unm_A6.250JuniorSubordinatedNotesdue2058Member_019f2411-8062-76c9-8eca-71b591cc5af5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_019f2411-8062-7db1-aaa5-fcac2c976c94" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_DocumentType_019f2411-8062-7db1-aaa5-fcac2c976c94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_019f2411-8062-7a30-8aa0-81e400c025be" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_DocumentPeriodEndDate_019f2411-8062-7a30-8aa0-81e400c025be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_019f2411-8062-7e0b-be70-1dbe98c0bd30" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityRegistrantName_019f2411-8062-7e0b-be70-1dbe98c0bd30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_019f2411-8062-770c-89fe-04af23a673ef" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityIncorporationStateCountryCode_019f2411-8062-770c-89fe-04af23a673ef" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_019f2411-8062-7fa7-a0f6-4e024fd75f3a" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityFileNumber_019f2411-8062-7fa7-a0f6-4e024fd75f3a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_019f2411-8062-7ea6-ab55-2d98db10d0aa" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityTaxIdentificationNumber_019f2411-8062-7ea6-ab55-2d98db10d0aa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_019f2411-8062-7265-bd78-59bd32a775db" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityAddressAddressLine1_019f2411-8062-7265-bd78-59bd32a775db" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_019f2411-8062-7aa9-8c73-eae2340ddef4" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityAddressCityOrTown_019f2411-8062-7aa9-8c73-eae2340ddef4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_019f2411-8062-7fa0-9642-f54d0e3087be" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityAddressStateOrProvince_019f2411-8062-7fa0-9642-f54d0e3087be" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_019f2411-8062-7d45-9665-f12ef2f6a3c8" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityAddressPostalZipCode_019f2411-8062-7d45-9665-f12ef2f6a3c8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_019f2411-8062-7c58-bf20-45b913f2a6f3" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_CityAreaCode_019f2411-8062-7c58-bf20-45b913f2a6f3" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_019f2411-8062-7958-a491-d1e4abecf8e1" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_LocalPhoneNumber_019f2411-8062-7958-a491-d1e4abecf8e1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_019f2411-8062-7e28-9cfb-f1e775a88b99" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_WrittenCommunications_019f2411-8062-7e28-9cfb-f1e775a88b99" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_019f2411-8062-7d43-87b4-6a35a3d62179" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_SolicitingMaterial_019f2411-8062-7d43-87b4-6a35a3d62179" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_019f2411-8062-7f32-af41-c75bc0ebad6e" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_PreCommencementTenderOffer_019f2411-8062-7f32-af41-c75bc0ebad6e" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_019f2411-8062-7d44-b37c-f1997a8295be" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityInformationLineItems_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_019f2411-8062-7d44-b37c-f1997a8295be" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_019f2411-8062-7735-9434-8679de61a3ae" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_Security12bTitle_019f2411-8062-7735-9434-8679de61a3ae" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_019f2411-8062-77a9-bbba-bb74a0e8378f" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_TradingSymbol_019f2411-8062-77a9-bbba-bb74a0e8378f" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_019f2411-8062-755b-ae9b-57329cc68d44" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_SecurityExchangeName_019f2411-8062-755b-ae9b-57329cc68d44" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_019f2411-8062-7303-8a82-3ee94c1263a7" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityEmergingGrowthCompany_019f2411-8062-7303-8a82-3ee94c1263a7" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_019f2411-8062-7d6b-9ec5-9369089a3eb5" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_EntityCentralIndexKey_019f2411-8062-7d6b-9ec5-9369089a3eb5" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_019f2411-8062-70fc-b5a8-0b06595ea98f" 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_019f2411-8062-76d3-9753-4c4a5eb36b29" xlink:to="loc_dei_AmendmentFlag_019f2411-8062-70fc-b5a8-0b06595ea98f" xlink:type="arc" order="21"/>
  </link:definitionLink>
</link:linkbase>
