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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0001140361-08-009561.txt : 20080415
<SEC-HEADER>0001140361-08-009561.hdr.sgml : 20080415
<ACCEPTANCE-DATETIME>20080415094140
ACCESSION NUMBER:		0001140361-08-009561
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20080411
FILED AS OF DATE:		20080415
DATE AS OF CHANGE:		20080415

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SENIOR HOUSING PROPERTIES TRUST
		CENTRAL INDEX KEY:			0001075415
		STANDARD INDUSTRIAL CLASSIFICATION:	REAL ESTATE INVESTMENT TRUSTS [6798]
		IRS NUMBER:				043445278
		STATE OF INCORPORATION:			MD
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		400 CENTRE STREET
		CITY:			NEWTON
		STATE:			MA
		ZIP:			02458
		BUSINESS PHONE:		6173323990

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			PORTNOY ADAM
		CENTRAL INDEX KEY:			0001273871

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-15319
		FILM NUMBER:		08756011

	MAIL ADDRESS:	
		STREET 1:		C/O REIT MANAGEMENT & RESEARCH
		STREET 2:		400 CENTRE ST
		CITY:			NEWTON
		STATE:			MA
		ZIP:			02458
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-04-11</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001075415</issuerCik>
        <issuerName>SENIOR HOUSING PROPERTIES TRUST</issuerName>
        <issuerTradingSymbol>NYSE: SNH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001273871</rptOwnerCik>
            <rptOwnerName>PORTNOY ADAM</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O REIT MANAGEMENT &amp; RESEARCH LLC</rptOwnerStreet1>
            <rptOwnerStreet2>400 CENTRE STREET</rptOwnerStreet2>
            <rptOwnerCity>NEWTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02458</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 Shares of Beneficial Interests</value>
            </securityTitle>
            <transactionDate>
                <value>2008-04-11</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>12290</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>23.43</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>38768</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Shares of Beneficial Interests</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>39019</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These shares were issued pursuant to an Advisory Agreement between the issuer and Reit Management &amp; Research LLC (&quot;RMR&quot;), of which Mr. Portnoy is a director and minority beneficial owner.  Mr. Portnoy purchased these shares from RMR upon their issuance.  Such transfers were exempt from Section 16 of the Securities Exchange Act of 1934, as amended (the &quot;Exchange Act&quot;), pursuant to Rule 16a-13 thereunder.</footnote>
        <footnote id="F2">In accordance with the Advisory Agreement referred to above, certain historical share price information was used to determine the number of common shares of beneficial interest paid to RMR.  The per share price amount listed above is the closing sale price of the issuer's common shares of beneficial interest on the New York Stock Exchange on April 11, 2008, when the shares were issued and purchased.</footnote>
        <footnote id="F3">Such shares are held by Reit Management &amp; Research LLC (&quot;RMR&quot;), of which Mr. Portnoy is a director and minority beneficial owner.  Mr. Portnoy disclaims beneficial interest of the shares held by RMR except to the extent of his pecuniary interest therein.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Adam P. Portnoy</signatureName>
        <signatureDate>2008-04-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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