<SUBMISSION>
<ACCESSION-NUMBER>0001110805-20-000012
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20200130
<FILING-DATE>20200203
<DATE-OF-FILING-DATE-CHANGE>20200203
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>Oliver Daniel S.
<CIK>0001456895
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>001-16417
<FILM-NUMBER>20570352
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>19003 IH-10 WEST
<CITY>SAN ANTONIO
<STATE>TX
<ZIP>78257
</MAIL-ADDRESS>
</REPORTING-OWNER>
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>NuStar Energy L.P.
<CIK>0001110805
<ASSIGNED-SIC>4610
<IRS-NUMBER>742956831
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>19003 IH-10 WEST
<CITY>SAN ANTONIO
<STATE>TX
<ZIP>78257
<PHONE>(210) 918-2000
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>19003 IH-10 WEST
<CITY>SAN ANTONIO
<STATE>TX
<ZIP>78257
</MAIL-ADDRESS>
<FORMER-COMPANY>
<FORMER-CONFORMED-NAME>VALERO L P
<DATE-CHANGED>20020110
</FORMER-COMPANY>
<FORMER-COMPANY>
<FORMER-CONFORMED-NAME>SHAMROCK LOGISTICS LP
<DATE-CHANGED>20000331
</FORMER-COMPANY>
</ISSUER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>wf-form4_158077992549960.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-01-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001110805</issuerCik>
        <issuerName>NuStar Energy L.P.</issuerName>
        <issuerTradingSymbol>NS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001456895</rptOwnerCik>
            <rptOwnerName>Oliver Daniel S.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>19003 IH 10 WEST</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN ANTONIO</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>78257</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP-Bus Dev &amp; Engineering</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
            </securityTitle>
            <transactionDate>
                <value>2020-01-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>12357</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>92282</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <transactionDate>
                <value>2020-01-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3553</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>28.20</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>88729</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On January 30, 2020, the Compensation Committee of the Board of Directors of NuStar GP, LLC, the general partner of the Issuer's general partner, determined that the performance units available to vest with respect to 2019 performance vested at 198%.</footnote>
        <footnote id="F2">Reflects common units not distributed to the reporting person in order to satisfy the reporting person's tax obligations or grants.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/Michelle S. Miller, as Attorney-in-Fact for Daniel S. Oliver</signatureName>
        <signatureDate>2020-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
