On August 29, 2013 the U.S. Department of the Treasury and the IRS ruled that same-sex couples, legally married in jurisdictions that recognized their marriages, would be treated as married for federal tax purposes. The ruling applies regardless of whether or not the jurisdiction in which the couple lives recognizes same-sex marriage.
The ruling was issued subsequent to the Supreme Court decision in United States v. Windsor (570 U.S. 744 (2013)). In this decision the court held that Section 3 of the Defense of Marriage Act of 1996 (P.L. 104-199) violated the equal protection clause of the U.S. Constitution.
Under the ruling, same-sex couples are treated as married for federal tax purposes for income taxes, gift taxes, and estate taxes. Affected are any aspects of a tax return where marriage is a factor, such as filing status, the amount of standard deduction, dependent exemptions, claiming refundable credits such as the earned income tax credit and the child tax credit, IRA contribution limits, and employee benefits.
Any same-sex marriage legally entered into in any state, the District of Columbia, a U.S. territory, or a foreign country is covered by the ruling. The ruling doesn't apply to registered domestic partnerships or civil unions recognized under state law.