<SUBMISSION>
<ACCESSION-NUMBER>0001596288-21-000004
<TYPE>4
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20210209
<FILING-DATE>20210210
<DATE-OF-FILING-DATE-CHANGE>20210210
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>del Alamo Jorge A.
<CIK>0001596288
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4
<ACT>34
<FILE-NUMBER>001-16417
<FILM-NUMBER>21614166
</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_161299217235861.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-02-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001596288</rptOwnerCik>
            <rptOwnerName>del Alamo Jorge A.</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>SVP and Controller</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
            </securityTitle>
            <transactionDate>
                <value>2021-02-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6527</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>16.0983</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>67137</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">1. This transaction was executed in multiple trades at prices ranging from $16.0900 to $16.1050 per unit. The price reported above reflects the weighed average purchase price. Upon request from the Securities and Exchange Commission, the issuer or unitholder of the issuer, full information regarding the number of units purchased at each separate price will be made available.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Michelle S. Miller, as Attorney-in-Fact for Jorge A. del Alamo</signatureName>
        <signatureDate>2021-02-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
