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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-04-04</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001171486</issuerCik>
        <issuerName>NATURAL RESOURCE PARTNERS LP</issuerName>
        <issuerTradingSymbol>NRP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001385281</rptOwnerCik>
            <rptOwnerName>Cline Christopher</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CLINE RESOURCE AND DEVELOPMENT COMP</rptOwnerStreet1>
            <rptOwnerStreet2>3801 PGA BOULEVARD, SUITE 903</rptOwnerStreet2>
            <rptOwnerCity>PALM BEACH GARDENS</rptOwnerCity>
            <rptOwnerState>FL</rptOwnerState>
            <rptOwnerZipCode>33410</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Controls entity with director</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
            </securityTitle>
            <transactionDate>
                <value>2013-04-04</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>756914</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4917548</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
            </securityTitle>
            <transactionDate>
                <value>2013-04-04</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>15138</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4917548</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On April 4, 2013, Cutlass Collieries LLC (&quot;Cutlass&quot;), an entity controlled by Christopher Cline (the &quot;Reporting Person&quot;), effected an in-kind distribution of 756,914 common units (the &quot;Distributed Units&quot;) to its direct and indirect owners, for no additional consideration, including the distribution of 15,138 of the Distributed Units to the Reporting Person. The Distributed Units had previously been indirectly beneficially owned by the Reporting Person through his being deemed to share voting and dispositive control over the common units held of record by Cutlass.</footnote>
        <footnote id="F2">The number of Common Units directly beneficially owned by the Reporting Person immediately after the distribution of the Distributed Units represents 5,659,324 Common Units beneficially owned by the Reporting Person immediately prior to the distribution of the Distributed Units (consisting of 4,902,410 Common Units owned directly by the Reporting Person and 756,914 Common Units owned indirectly by the Reporting Person by virtue of his control over Cutlass, the record owner of such Common Units), less the distribution of the Distribution Units, plus 15,138 of the Distributed Units that were distributed to the Reporting Person as set forth in footnote (1) above.</footnote>
    </footnotes>

    <remarks>Adena Minerals, LLC (&quot;Adena&quot;), a company controlled by the Reporting Person, is entitled to nominate two directors to the board of GP Natural Resource Partners LLC, the general partner of NRP (GP) LP, which is the general partner of the Company.</remarks>

    <ownerSignature>
        <signatureName>/s/ Christopher Cline</signatureName>
        <signatureDate>2013-04-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
