<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>2110100 - Disclosure - DERIVATIVE FINANCIAL INSTRUMENTS</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>D2013Q2YTD</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000899751</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Derivative Instruments and Hedging Activities Disclosure [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="D2013Q2YTD" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family:Times New Roman;font-size:10pt;"&gt;&lt;div style="line-height:120%;text-align:left;font-size:10pt;"&gt;&lt;font style="font-family:inherit;font-size:10pt;font-weight:bold;"&gt; DERIVATIVE FINANCIAL INSTRUMENTS&lt;/font&gt;&lt;/div&gt;&lt;div style="line-height:120%;text-align:left;font-size:10pt;"&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;&lt;br clear="none"/&gt;&lt;/font&gt;&lt;/div&gt;&lt;div style="line-height:120%;text-align:left;font-size:10pt;"&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;Prior to the April 2013 payoff of its Term Loan with Bank of America, N.A. (BoA Term Loan), the Company used financial derivatives to mitigate its exposure to volatility in the interest rate and foreign currency exchange rate in Brazil.  The Company used these derivative instruments to hedge exposure in the ordinary course of business and did not invest in derivative instruments for speculative purposes.  In order to reduce interest rate and foreign currency risk on the BoA Term Loan, the Company entered into an interest rate swap agreement and cross currency swap transactions with Bank of America Merrill Lynch Banco Multiplo S.A. that was designed to convert the outstanding &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$5.0 million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; US Dollar based LIBOR loan to a Brazilian Real based CDI loan.  The Company did not designate these agreements as a hedging instrument.  Changes in the fair value of the cross currency swap were recorded in other income (expense) and changes in the fair value of the interest rate swap agreement were recorded as interest expense (or gain as an offset to interest expense).  For the three months ended &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;June&amp;#160;30, 2013&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;, the Company recorded &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$(0.5) million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; of other expense and &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$0.0 million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; of interest expense related to these derivatives.  For the six months ended &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;June&amp;#160;30, 2013&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;, the Company recorded &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$(0.6) million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; of other expense and &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$0.1 million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; of interest expense related to these derivatives. &lt;/font&gt;&lt;/div&gt;&lt;div style="line-height:120%;text-align:left;font-size:10pt;"&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;&lt;br clear="none"/&gt;&lt;/font&gt;&lt;/div&gt;&lt;div style="line-height:120%;text-align:left;font-size:10pt;"&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;The Company also used derivative financial instruments to manage its exposure to market risks from changes in interest rates in Europe.  These derivative financial instruments are recognized at fair value.  The Company has not designated these financial instruments as hedging instruments.  Any gain or loss on the re-measurement of the fair value is taken to interest expense.  For the three months ended &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;June&amp;#160;30, 2013&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;, the Company recorded interest expense of &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$0.0 million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; related to these derivatives.  For the six months ended &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;June&amp;#160;30, 2013&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt;, the Company recorded an offset to interest expense of &lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;color:#000000;text-decoration:none;"&gt;$1.1 million&lt;/font&gt;&lt;font style="font-family:inherit;font-size:10pt;"&gt; related to these derivatives.&lt;/font&gt;&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for the entity's entire derivative instruments and hedging activities.  Describes an entity's risk management strategies, derivatives in hedging activities and non-hedging derivative instruments, the assets, obligations, liabilities, revenues and expenses arising therefrom, and the amounts of and methodologies and assumptions used in determining the amounts of such items.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 30

 -Section 50

 -Paragraph 1

 -URI http://asc.fasb.org/extlink&amp;oid=7668309&amp;loc=d3e80748-113994



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=d3e41638-113959



Reference 3: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4E

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5624181-113959



Reference 4: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 3

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=d3e41635-113959



Reference 5: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 30

 -Section 45

 -Paragraph 1

 -URI http://asc.fasb.org/extlink&amp;oid=6441202&amp;loc=d3e80720-113993



Reference 6: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4J

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5708773-113959



Reference 7: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4H

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5624258-113959



Reference 8: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section S99

 -Paragraph 1

 -Subparagraph (SX 210.4-08.(n))

 -URI http://asc.fasb.org/extlink&amp;oid=6881521&amp;loc=d3e23780-122690



Reference 9: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4A

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5618551-113959



Reference 10: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4B

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5624163-113959



Reference 11: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Statement of Financial Accounting Standard (FAS)

 -Number 133

 -Paragraph 44

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 12: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4K

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5708775-113959



Reference 13: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 25

 -Section 50

 -Paragraph 1

 -URI http://asc.fasb.org/extlink&amp;oid=6886632&amp;loc=d3e76258-113986



Reference 14: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Statement of Financial Accounting Standard (FAS)

 -Number 133

 -Paragraph 45

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 15: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 30

 -Section 50

 -Paragraph 2

 -URI http://asc.fasb.org/extlink&amp;oid=7668309&amp;loc=d3e80784-113994



Reference 16: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=d3e41620-113959



Reference 17: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 1B

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5580258-113959



Reference 18: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 1A

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5579245-113959



Reference 19: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 1

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5579240-113959



Reference 20: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 5

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=d3e41641-113959



Reference 21: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4C

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5624171-113959



Reference 22: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 815

 -SubTopic 10

 -Section 50

 -Paragraph 4D

 -URI http://asc.fasb.org/extlink&amp;oid=7476318&amp;loc=SL5624177-113959



</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>DERIVATIVE FINANCIAL INSTRUMENTS</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>DERIVATIVE FINANCIAL INSTRUMENTS</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.titan-intl.com/role/DerivativeFinancialInstruments</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
