To file a joint tax return, you must be married or considered married on the last day of the year. If you haven't been officially married, the only way to meet this requirement is to meet the requirements for a common law marriage. Otherwise, you're both stuck filing single returns.
Common Law Marriages
If you are living in a state that recognizes your relationship as a common law marriage, the IRS recognizes it, too, and you can file a joint return. In addition, you can also file a joint return if your common law marriage was recognized in the state the marriage began in.
Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."