Reuters/Joshua Roberts
Swing voter Justice Anthony Kennedy's opinion finds
The opinion says DOMA "contrives to deprive some couples married under the laws of their State, but not others, of both rights and responsibilities, creating two contradictory marriage regimes within the same state."
The case was brought by Edith Windsor, an 83-year-old lesbian who paid $363,000 in estate taxes when her wife died that a heteroxexual spouse wouldn't have had to pay.
Bill Clinton signed DOMA in 1996 after Hawaii signaled that it might soon legalize gay marriage. Section 3 of DOMA says the federal government doesn't recognize gay marriage sanctioned by states.
The law - which even Clinton has renounced - effectively deprives same-sex married couples of many of the benefits that straight couples get, including tax breaks and the ability to help spouses immigrate to the U.S.
Two appeals courts have found that DOMA violates the right to equal protection under the Constitution.