A crowd of thousands gathered in Washington to celebrate the Supreme Court striking down a part of a federal law denying federal benefits to married gay couples.
The court's 5-4 vote said the Defence of Marriage Act, known as Doma, denied equal protection to same-sex couples.
The ruling means that previous anti-gay marriage laws denying legally married same-sex couples from receiving tax, health and pension benefits has been swept away.
The court also upheld a trial court declaration that ruled that California's Proposition 8 was unconstitutional, paving the way for same-sex marriages to continue in the state.
Proposition 8 was approved in 2008 with 52% of the vote and put same-sex marriages on hold.
The verdict read:
"Under Doma, same-sex married couples have their lives burdened, by reason of government decree, in visible and public ways.
"Doma's principal effect is to identify a subset of state-sanctioned marriages and make them unequal."
The US Supreme Court has ruled married gay couples should be entitled to the same benefits as heterosexual couples.
The US Supreme Court will this week begin a new chapter in its review of discrimination based on sexual orientation.