The court also dismissed a case on California's ban on gay marriage, effectively reinstating marriage rights for gays and lesbians in the state.
Although there were scenes of celebration on the steps of the Supreme Court in Washington there was also realism. As one gay-rights leader said, there will be "two Americas" and legal complications for many gay couples moving between them.
The court's rulings have no direct effect on the constitutional amendments in 29 states that limit marriage to heterosexual couples. In a handful of politically moderate states such as Oregon, Nevada and Colorado, those amendments could be overturned by ballot measures but that's considered highly unlikely in more conservative states.
"It would be inefficient to try to pick off 30 constitutional amendments one by one," said Fred Sainz of the Human Rights Campaign, a national gay-rights group. "Eventually this will have to be settled by the Supreme Court."
The Campaign's president, Chad Griffin, said the goal would be to legalise same-sex marriage nationwide within five years.
- additional reporting, AP
'Torch of freedom'
The case that led to the act's downfall was known as United States v Windsor, after Edith Windsor (right), an 84-year-old lesbian who sued the federal government after she was made to pay US$363,000 ($464,000) in taxes after the death of her partner of 44 years. "Edie has lit a torch of freedom for generations of Americans to follow," said her lawyer, Roberta Kaplan.