"DOMA's avowed purpose and practical effect are to impose a disadvantage, a separate status, and so a stigma upon all who enter into same-sex marriages made lawful by the unquestioned authority of the States," Kennedy wrote in an opinion that was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.
Crowds outside the Supreme Court could be heard erupting in applause as news of the court's ruling spread.
Justice Antonin Scalia, who dissented along with conservative Justices John Roberts, Clarence Thomas, and Samuel Alito, read his dissent from the bench.
He said that he doesn't believe the court had the jurisdiction to hear the case.
"There are two parts to the majority's opinion, the first explaining why this Court has jurisdiction to decide the question, and the second deciding it," Scalia said. "Both of them are wrong, and the error in both springs from the same diseased root: an exalted notion of the role of this Court in American democratic society." Read ABC News' live blog of the Supreme Court's decisions HERE.