The ruling in the California case was not along ideological lines. Chief Justice John Roberts wrote the majority opinion, joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Elena Kagan and Antonin Scalia.
"We have no authority to decide this case on the merits, and neither did the 9th Circuit," Roberts said, referring to the federal appeals court that also struck down Proposition 8.
In the case involving the federal Defense of Marriage Act, Justice Anthony Kennedy wrote the majority opinion, joined by the court's liberal justices.
"Under DOMA, same-sex married couples have their lives burdened, by reason of government decree, in visible and public ways," Kennedy said.
"DOMA's principal effect is to identify a subset of state-sanctioned marriages and make them unequal," he said.
Some in the crowd outside the court hugged and others jumped up and down just after 10 a.m. EDT Wednesday when the DOMA decision was announced. Many people were on their cell phones monitoring Twitter, news sites and blogs for word of the decision. And there were cheers as runners came down the steps with the decision in hand and turned them over to reporters who quickly flipped through the decisions.
Chants of "Thank you" and "USA" came from the crowd as plaintiffs in the cases descended the court's marbled steps. Most of those in the crowd appeared to support gay marriage, although there was at least one man who held a sign promoting marriage as between a man and a woman.
Kennedy was joined in the DOMA decision by the court's four liberal justices.
Chief Justice John Roberts, Justices Samuel Alito and Clarence Thomas, and Scalia dissented.
Same-sex marriage has been adopted by 12 states and the District of Columbia. Another 18,000 couples were married in California during a brief period when same-sex unions were legal there.