<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>1120 - Disclosure - Related Party Transactions</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2010 - 12/31/2010
USD ($)

USD ($) / shares

</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>D2010</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001063761</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2010-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2010-12-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS><UnitProperty><UnitID>USD</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>USDPerShare</UnitID><UnitType>Divide</UnitType><NumeratorMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></NumeratorMeasure><DenominatorMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></DenominatorMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>Pure</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>pure</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>Shares</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="12 Months Ended" /><Label Id="2" Label="Dec. 31, 2010" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><Level>0</Level><ElementName>spg_RelatedPartyTransactionsDisclosureAbstract</ElementName><ElementPrefix>spg</ElementPrefix><IsBaseElement>false</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole /><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>No definition available.</ElementDefenition><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Related Party Transactions</Label></Row><Row><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_RelatedPartyTransactionsDisclosureTextBlock</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terselabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;table style="font-size:10pt; font-family:'Times New Roman',times,serif;"&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&lt;b&gt;12.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Related Party Transactions &lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Our management company provides management, insurance, and other services to Melvin Simon&amp;nbsp;&amp;amp; Associates,&amp;nbsp;Inc., a related party, and other non-owned properties. Amounts for services provided by our management company and its affiliates to our unconsolidated joint ventures and other related parties were as follows: &lt;/font&gt;&lt;/p&gt;
&lt;div style="PADDING-RIGHT: 0pt; PADDING-LEFT: 0pt; PADDING-BOTTOM: 0pt; MARGIN-LEFT: 20%; WIDTH: 60%; PADDING-TOP: 0pt; POSITION: relative"&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&lt;!-- COMMAND=ADD_TABLEWIDTH,"100%" --&gt;&lt;/font&gt;&lt;/p&gt;
&lt;!-- User-specified TAGGED TABLE --&gt;
&lt;div align="center"&gt;
&lt;table cellspacing="0" cellpadding="0" width="100%" border="0"&gt;
&lt;tr style="HEIGHT: 0px"&gt;&lt;!-- TABLE COLUMN WIDTHS SET --&gt;
&lt;td style="FONT-FAMILY: times" align="left"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right" width="6"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="46"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right" width="6"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="46"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right" width="6"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="46"&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" width="12"&gt;&lt;/td&gt;&lt;!-- TABLE COLUMN WIDTHS END --&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom"&gt;
&lt;th style="FONT-FAMILY: times" align="left"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;br /&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;
&lt;th style="BORDER-BOTTOM: #000000 1pt solid; FONT-FAMILY: times" align="center" colspan="8"&gt;&lt;font size="1"&gt;&lt;b&gt;For the Year Ended December&amp;nbsp;31, &lt;/b&gt;&lt;/font&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom"&gt;
&lt;th style="FONT-FAMILY: times" align="left"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;br /&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;
&lt;th style="BORDER-BOTTOM: #000000 1pt solid; FONT-FAMILY: times" align="center" colspan="2"&gt;&lt;font size="1"&gt;&lt;b&gt;2010 &lt;/b&gt;&lt;/font&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;
&lt;th style="BORDER-BOTTOM: #000000 1pt solid; FONT-FAMILY: times" align="center" colspan="2"&gt;&lt;font size="1"&gt;&lt;b&gt;2009 &lt;/b&gt;&lt;/font&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;
&lt;th style="BORDER-BOTTOM: #000000 1pt solid; FONT-FAMILY: times" align="center" colspan="2"&gt;&lt;font size="1"&gt;&lt;b&gt;2008 &lt;/b&gt;&lt;/font&gt;&lt;/th&gt;
&lt;th style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/th&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="#CCEEFF"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;Amounts charged to unconsolidated joint ventures&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&lt;b&gt;$&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;&lt;b&gt;118,905&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;120,866&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;125,663&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="HEIGHT: 0px" valign="bottom" bgcolor="white"&gt;
&lt;td style="FONT-FAMILY: times"&gt;
&lt;p style="MARGIN-LEFT: 9pt; TEXT-INDENT: -9pt; FONT-FAMILY: times"&gt;&lt;font size="2"&gt;Amounts charged to properties owned by related parties&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;&lt;b&gt;4,308&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;4,522&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times" align="right"&gt;&lt;font size="2"&gt;4,980&lt;/font&gt;&lt;/td&gt;
&lt;td style="FONT-FAMILY: times"&gt;&lt;font size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;
&lt;!-- end of user-specified TAGGED TABLE --&gt;&lt;/div&gt;
&lt;p style="FONT-FAMILY: times; TEXT-ALIGN: justify"&gt;&lt;font size="2"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;During 2010, 2009 and 2008, we recorded interest income of $9.9&amp;nbsp;million, $9.3&amp;nbsp;million and $15.3&amp;nbsp;million respectively, and financing fee income of $0.9&amp;nbsp;million, $3.7&amp;nbsp;million and $3.1&amp;nbsp;million, respectively, net of inter-entity eliminations, related to the loans that we have provided to Mills and SPG-FCM and lending financing services to those entities and the properties in which they hold an ownership interest. &lt;/font&gt;&lt;/p&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;
</NonNumbericText><NonNumericTextHeader>12.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Related Party Transactions
&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Our management company provides</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>This element may be used for the entire related party transactions disclosure as a single block of text. Disclosure may include:  the nature of the relationship(s), a description of the transactions, the amount of the transactions, the effects of any change in the method of establishing the terms of the transaction from the previous period, stated interest rate, expiration date, terms and manner of settlement per the agreement with the related party, and amounts due to or from related parties. If the entity and one or more other entities are under common ownership or management control and this control affects the operating results or financial position, disclosure includes the nature of the control relationship even if there are no transactions between the entities. Disclosure may also include the aggregate amount of current and deferred tax expense for each statement of earnings presented where the entity is a member of a group that files a consolidated tax return, the amount of any tax related balances due to or from affiliates as of the date of each statement of financial position presented, the principal provisions of the method by which the consolidated amount of current and deferred tax expense is allocated to the members of the group and the nature and effect of any changes in that method. Examples of related party transactions include transactions between (a) a parent company and its subsidiary; (b) subsidiaries of a common parent; (c) and entity and its principal owners; and (d) affiliates.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 04
 -Paragraph b
 -Article 3A

Reference 2: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 08
 -Paragraph k
 -Article 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name Statement of Financial Accounting Standard (FAS)
 -Number 57
 -Paragraph 1-4

</ElementReferences><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Related Party Transactions</Label></Row></Rows><Footnotes /><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows><ReportName>Related Party Transactions</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>false</HasCustomUnits><SharesShouldBeRounded>true</SharesShouldBeRounded></InstanceReport>
