-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 APHv3IEmLowgM1uDM91FEbyk5Ge4I+bJnR+DNsy/8TbVc18LwDeV5Hrn8svCBOub
 1dScZEoxLFZ1Hft5lH/IZA==

<SEC-DOCUMENT>0000944695-10-000058.txt : 20101001
<SEC-HEADER>0000944695-10-000058.hdr.sgml : 20101001
<ACCEPTANCE-DATETIME>20101001092316
ACCESSION NUMBER:		0000944695-10-000058
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100930
FILED AS OF DATE:		20101001
DATE AS OF CHANGE:		20101001

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			HUBER J KENDALL
		CENTRAL INDEX KEY:			0001227471

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-13754
		FILM NUMBER:		101100802

	MAIL ADDRESS:	
		STREET 1:		C/O THE HANOVER INSURANCE GROUP, INC.
		STREET 2:		440 LINCOLN ST. (E-10)
		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:			1231

	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>2010-09-30</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001227471</rptOwnerCik>
            <rptOwnerName>HUBER J KENDALL</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE HANOVER INSURANCE GROUP, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>440 LINCOLN ST. (E-10)</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>Senior Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-09-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>44.05</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>59771</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-09-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>47</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>43271</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>44.05</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2010-09-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>16500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2012-01-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>16500</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16500</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on 2/26/10.</footnote>
        <footnote id="F2">Fully exercisable on 01/02/05.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Walter H. Stowell pursuant to Confirming Statement</signatureName>
        <signatureDate>2010-10-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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