-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 MRTZZM0oxTiLUwcft6wMTmUBC4uvmHyaUqmHJulbUtaRi4tRzNVzLnwIXdNw9EDw
 kHaXH3G1tEld+MWoU46sNg==

<SEC-DOCUMENT>0001179110-09-013936.txt : 20091001
<SEC-HEADER>0001179110-09-013936.hdr.sgml : 20091001
<ACCEPTANCE-DATETIME>20091001171855
ACCESSION NUMBER:		0001179110-09-013936
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090929
FILED AS OF DATE:		20091001
DATE AS OF CHANGE:		20091001

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			VIEAU DAVID P
		CENTRAL INDEX KEY:			0001194667

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-34463
		FILM NUMBER:		091099515

	MAIL ADDRESS:	
		STREET 1:		4991 CORPORATE DRIVE
		CITY:			HUNTVILLE
		STATE:			AL
		ZIP:			35805

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			A123 SYSTEMS, INC.
		CENTRAL INDEX KEY:			0001167178
		STANDARD INDUSTRIAL CLASSIFICATION:	MISCELLANEOUS ELECTRICAL MACHINERY, EQUIPMENT & SUPPLIES [3690]
		IRS NUMBER:				043583876
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		ARSENAL ON THE CHARLES
		STREET 2:		321 ARSENAL STREET
		CITY:			WATERTOWN
		STATE:			MA
		ZIP:			02472
		BUSINESS PHONE:		6177785700

	MAIL ADDRESS:	
		STREET 1:		ARSENAL ON THE CHARLES
		STREET 2:		321 ARSENAL STREET
		CITY:			WATERTOWN
		STATE:			MA
		ZIP:			02472

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	A123 SYSTEMS INC
		DATE OF NAME CHANGE:	20020212
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-09-29</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001167178</issuerCik>
        <issuerName>A123 SYSTEMS, INC.</issuerName>
        <issuerTradingSymbol>AONE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001194667</rptOwnerCik>
            <rptOwnerName>VIEAU DAVID P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O A123 SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>321 ARSENAL STREET</rptOwnerStreet2>
            <rptOwnerCity>WATERTOWN</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02472</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Executive Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-09-29</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>186485</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>13.5</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>998515</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes></footnotes>

    <ownerSignature>
        <signatureName>/s/ Eric J. Pyenson, attorney-in-fact</signatureName>
        <signatureDate>2009-10-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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