Yes, but only if you meet the IRS' five-point test.
Only couples legally married on December 31 of the tax year can file joint income tax returns. But if one partner supports the other, the supporter can file a tax return as a single person and claim their partner as a dependent. Here is the test:
Are They Unmarried?
If the supported person is married, filing a joint tax return with his spouse (unusual in your situation), the supporting partner in this relationship cannot claim him as a dependent. The one exception to this: if the married couple did not earn enough to have to file a tax return, and filed only to get a refund, the supporting partner can still claim the dependent.
Are They A Citizen or Resident Alien?
The supported person must be a U.S. citizen, resident alien or a citizen of Canada or Mexico.
Is Their Income Low Enough?
The supported person's taxable income cannot exceed $2,900. (Certain nontaxable money, such as gifts, welfare benefits and nontaxable Social Security benefits, don't count toward gross income.)
Do You Provide Their Support?
The supporting partner must provide at least 50% of the other partner's total support for the year in order to claim them as a dependent. Support includes food, shelter, clothing, medical and dental care, education, entertainment and just about anything similar.
Is Your Relationship Legal?
Under IRS regulations, a person who lives with you for the entire year can be considered a dependent so long as the relationship does not violate local law. The IRS has not been explicit on whether this is a reference to laws prohibiting certain sexual acts. Conclusion: If you meet the other four tests, go ahead and claim your partner as a dependent. The worst that can happen is that the IRS won't allow your deduction and your tax bill will be recomputed without the deduction. As Wayne Gretzky said, you miss 100% of the shots you never take.