<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0205</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2012-04-01</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001358831</issuerCik>
        <issuerName>LEGACY RESERVES LP</issuerName>
        <issuerTradingSymbol>LGCY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001544888</rptOwnerCik>
            <rptOwnerName>Foster Micah C.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>303 W. WALL, SUITE 1400</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MIDLAND</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>79701</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CAO and Controller</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Units representing limited partner interests</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4000</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Unit Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>20.16</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-08-26</value>
            </exerciseDate>
            <expirationDate>
                <value>2013-08-26</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Units representing limited partner interests</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3333</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Unit Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>16.07</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-08-20</value>
            </exerciseDate>
            <expirationDate>
                <value>2016-08-20</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Units representing limited partner interests</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3333</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Unit Appreciation Rights</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>24.03</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-08-16</value>
            </exerciseDate>
            <expirationDate>
                <value>2017-08-16</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Units representing limited partner interests</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3333</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted units vest annually in one-third increments beginning on May 19, 2012.</footnote>
    </footnotes>

    <remarks>Chief Accounting Officer and Controller of Legacy Reserves GP, LLC, the general partner of Legacy Reserves LP.</remarks>

    <ownerSignature>
        <signatureName>/s/ Micah C. Foster</signatureName>
        <signatureDate>2012-04-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
