<SUBMISSION>
<ACCESSION-NUMBER>0001349304-20-000003
<TYPE>4/A
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20190916
<FILING-DATE>20200213
<DATE-OF-FILING-DATE-CHANGE>20200213
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>Barron Bradley C
<CIK>0001349304
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>4/A
<ACT>34
<FILE-NUMBER>001-16417
<FILM-NUMBER>20611350
</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/A
<SEQUENCE>1
<FILENAME>wf-form4a_158162990624996.xml
<DESCRIPTION>FORM 4/A
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2019-09-16</periodOfReport>

    <dateOfOriginalSubmission>2019-11-19</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001349304</rptOwnerCik>
            <rptOwnerName>Barron Bradley C</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>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>CEO &amp; President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Units</value>
            </securityTitle>
            <transactionDate>
                <value>2019-09-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1700</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>204033</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This filing is being amended to correct the number of common units gifted from 17,000 to 1,700.</footnote>
        <footnote id="F2">The number reported in Box 5 above corrects the number of common units beneficially owned by Mr. Barron as of the transaction date, September 16, 2019. Subsequent reports on Form 4 have been filed on behalf of Mr. Barron since the original filing date of this report. As of February 13, 2020, the number of units beneficially owned by Mr. Barron is 306,626.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Michelle S. Miller, as Attorney-in-Fact for Bradley C. Barron</signatureName>
        <signatureDate>2020-02-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
