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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,
 Q1R5fzIqK1Q9Qyf/Qq0dQQ0dUJz91DCxjSWDXQ/wQszL+p8sJagkh1RtkvFEV1eP
 0vsSnU2jEKOKFiY6IzDzmQ==

<SEC-DOCUMENT>0000944695-09-000010.txt : 20090218
<SEC-HEADER>0000944695-09-000010.hdr.sgml : 20090218
<ACCEPTANCE-DATETIME>20090218164801
ACCESSION NUMBER:		0000944695-09-000010
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090217
FILED AS OF DATE:		20090218
DATE AS OF CHANGE:		20090218

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			FIRSTENBERG DAVID J
		CENTRAL INDEX KEY:			0001269363

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-13754
		FILM NUMBER:		09619100

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		5088556404

	MAIL ADDRESS:	
		STREET 1:		ALLMERICA FINANCIAL CORP
		STREET 2:		440 LINCOLN STREET
		CITY:			WORCESTER
		STATE:			MA
		ZIP:			01653

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			HANOVER INSURANCE GROUP, INC.
		CENTRAL INDEX KEY:			0000944695
		STANDARD INDUSTRIAL CLASSIFICATION:	FIRE, MARINE & CASUALTY INSURANCE [6331]
		IRS NUMBER:				043263626
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1106

	BUSINESS ADDRESS:	
		STREET 1:		440 LINCOLN ST
		CITY:			WORCESTER
		STATE:			MA
		ZIP:			01653
		BUSINESS PHONE:		5088551000

	MAIL ADDRESS:	
		STREET 1:		440 LINCOLN ST
		CITY:			WORCESTER
		STATE:			MA
		ZIP:			01653

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	ALLMERICA FINANCIAL CORP
		DATE OF NAME CHANGE:	19950501
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-02-17</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000944695</issuerCik>
        <issuerName>HANOVER INSURANCE GROUP, INC.</issuerName>
        <issuerTradingSymbol>THG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001269363</rptOwnerCik>
            <rptOwnerName>FIRSTENBERG DAVID J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>THE HANOVER INSURANCE GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>440 LINCOLN STREET</rptOwnerStreet2>
            <rptOwnerCity>WORCESTER</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>01653</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-02-17</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>988</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.66</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23731</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Does not include approximately 5,262 shares held indirectly by 401(k)</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Walter H. Stowell pursuant to Confirming Statement</signatureName>
        <signatureDate>2009-02-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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