The court also knocked back a separate appeal against same-sex marriage laws in California, restoring the right to gay marriage in the largest US state and nearly doubling the number of Americans living in states where gay marriage would be legal.
Together, the two Wednesday rulings mark the biggest advance in civil liberties for gay people in a generation, and come amid growing political and international recognition that same-sex couples deserve equal legal treatment.
The most significant legal breakthrough came in the decision led by Anthony Kennedy to rule that the Defense of Marriage Act (Doma) was unconstitutional because it deprived citizens of "equal liberty" before the law.