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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-03-06</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001676601</rptOwnerCik>
            <rptOwnerName>Baines Creek Capital, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>11940 JOLLYVILLE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 210-S</rptOwnerStreet2>
            <rptOwnerCity>AUSTIN</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>78759</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2018-03-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>150000</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.13</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12163357</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2018-03-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>45000</value>
                    <footnoteId id="F5"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.03</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12208357</value>
                    <footnoteId id="F6"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common</value>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2018-03-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>79000</value>
                    <footnoteId id="F7"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>4.1</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12287357</value>
                    <footnoteId id="F8"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This Form 4 is being jointly filed by Baines Creek Partners, L.P., a Delaware limited partnership (BCP), Baines Creek Special Purpose Partners, L.P., a Delaware limited partnership (BCSPP), Kevin Tracy, a United States citizen (KT), Jeremy Carter a United States citizen (JC), James Schumacher, a United States citizen (JS), and Brian Williams a United States citizen (BW). The foregoing persons are hereinafter sometimes collectively referred to as the Reporting Persons.</footnote>
        <footnote id="F2">Baines Creek GP the general partner of BCP and BCSPP has the power to direct the affairs of BCP and BCSPP, including the voting and disposition of shares. As the discretionary investment manager of these Funds and certain other accounts, Baines Creek Capital has power to direct the voting and disposition of shares held by these Funds and such accounts. Messrs. Tracy, Carter, Schumacher and Williams are affiliated persons of Baines Creek Capital and in this capacity direct the operations of BCP and BCSPP and the voting and disposition of shares held by these Funds and separate client accounts managed by Baines Creek Capital. Baines Creek Capital and Messrs. Tracy, Carter, Schumacher and Williams may also be deemed to beneficially own indirect pecuniary interest as the result of performance-based fees and profit allocations. Each of Baines Creek Capital and Messrs. Tracy, Carter, Schumacher and Williams disclaims such beneficial ownership except to the extent ultimately realized.</footnote>
        <footnote id="F3">The aggregate number of shares of common stock purchased on March 6, 2018 was 150,000 shares and such shares were purchased by the Reporting Persons in the following amounts: BCP = 0   shares; BCSPP = 150,000 shares; KT = 0 shares; JC = 0 shares; JS = 0 shares, and BW = 0 shares.</footnote>
        <footnote id="F4">12,163,357 shares of common stock is the aggregate number of shares of common stock owned by the Reporting Persons and is owned as follows: BCP = 6,500,000 shares directly owned by it; BCSPP = 5,036,472 shares directly owned by it; KT = 6,510 shares directly owned by him; JC = 142,317 shares directly owned by him; JS = 4,686 shares directly owned by him, and BW = 473,372 shares directly owned by him.</footnote>
        <footnote id="F5">The aggregate number of shares of common stock purchased on March 7, 2018 was 45,000 shares and such shares were purchased by the Reporting Persons in the following amounts: BCP = 0   shares; BCSPP = 45,000 shares; KT = 0 shares; JC = 0 shares; JS = 0 shares, and BW = 0 shares.</footnote>
        <footnote id="F6">12,208,357 shares of common stock is the aggregate number of shares of common stock owned by the Reporting Persons and is owned as follows: BCP = 6,500,000 shares directly owned by it; BCSPP = 5,081,472 shares directly owned by it; KT = 6,510 shares directly owned by him; JC = 142,317 shares directly owned by him; JS = 4,686 shares directly owned by him, and BW = 473,372 shares directly owned by him.</footnote>
        <footnote id="F7">The aggregate number of shares of common stock purchased on March 8, 2018 was 79,000 shares and such shares were purchased by the Reporting Persons in the following amounts: BCP = 0   shares; BCSPP = 79,000 shares; KT = 0 shares; JC = 0 shares; JS = 0 shares, and BW = 0 shares.</footnote>
        <footnote id="F8">12,287,357 shares of common stock is the aggregate number of shares of common stock owned by the Reporting Persons and is owned as follows: BCP = 6,500,000 shares directly owned by it; BCSPP = 5,160,472 shares directly owned by it; KT = 6,510 shares directly owned by him; JC = 142,317 shares directly owned by him; JS = 4,686 shares directly owned by him, and BW = 473,372 shares directly owned by him.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/S/ Brian Williams</signatureName>
        <signatureDate>2018-03-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
