The Supreme Court on Wednesday struck down a central portion of a federal law that restricted the definition of marriage to opposite-sex couples in a major victory for the gay rights movement.
The ruling, on a 5-4 vote, means that legally married gay men and women are entitled to claim the same federal benefits that are available to opposite-sex married couples.
The court was due to decide within minutes a second case concerning a California law that bans same-sex marriage in the state.
Justice Anthony Kennedy wrote in the majority opinion that the Defense of Marriage Act violates the U.S. Constitution's guarantee of equal protection.
"The federal statute is invalid, for no legitimate purpose overcomes the purpose and effect to disparage and to injure those whom the state, by its marriage laws, sought to protect in personhood and dignity," Kennedy wrote.
Kennedy, often the court's swing vote in close decisions, also said the law imposes "a stigma upon all who enter into same-sex marriages made lawful by the unquestioned authority of the states."
Chief Justice John Roberts and Justice Antonin Scalia both wrote dissenting opinions.
In his dissent, Roberts wrote that the court in the coming Proposition 8 case will not reach the issue of constitutionality of state laws that limit the definition of marriage.
By striking down Section 3 of the law, the court clears the way to more than 1,100 federal benefits, rights and burdens linked to marriage status.
As a result of Wednesday's ruling, Edith Windsor of New York, who was married to a woman and sued the government to get the federal estate tax deduction available to heterosexuals when their spouses die, will be able to claim a $363,000 tax refund.