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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-08-07</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001409375</issuerCik>
        <issuerName>ORION ENERGY SYSTEMS, INC.</issuerName>
        <issuerTradingSymbol>OESX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001522618</rptOwnerCik>
            <rptOwnerName>Jacobson Tryg C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>529 ONTARIO AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SHEBOYGAN</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>53081</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-08-07</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1912</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>54223</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>3.12</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2012-08-09</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-08-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>22045</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22045</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.03</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-06-12</value>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-06-12</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>26646</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26646</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Options (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.41</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2013-12-31</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2023-05-28</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>36568</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>36568</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Shares granted in lieu of cash for quarterly retainer and annual Committee Chairman fees pursuant to Issuer's 2004 Stock and Incentive Awards Plan.</footnote>
        <footnote id="F2">Grant to reporting person of option to buy shares under the 2004 Stock and Incentive Awards Plan. This option, granted August 9, 2011, vests and becomes exercisable in three equal installments on August 9, 2012, 2013 and 2014, respectively.</footnote>
        <footnote id="F3">Grant to reporting person of option to buy shares under the 2004 Stock and Incentive Awards Plan. This option, granted June 12, 2012, vests and becomes exercisable in three equal installments on June 12, 2013, 2014 and 2015, respectively.</footnote>
        <footnote id="F4">Grant to reporting person of option to buy shares under the 2004 Stock and Incentive Awards Plan. This option, granted May 28, 2013, vests and becomes exercisable in three equal installments on December 31, 2013, 2014 and 2015, respectively.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Steven R. Barth, Attorney-in-Fact for Tryg C. Jacobson</signatureName>
        <signatureDate>2015-01-15</signatureDate>
    </ownerSignature>
</ownershipDocument>
