Justice Anthony Kennedy wrote that the plaintiffs asked "for equal dignity in the eyes of the law. The Constitution grants them that right." The ruling brings to an end more than a decade of bitter legal battles. Same-sex couples in several affected states including Georgia, Michigan, Ohio and Texas rushed to wed on Friday.
However officials in other states, including Mississippi and Louisiana, said marriages had to wait until procedural issues were addressed.
President Barack Obama said the ruling was a "victory for America". "When all Americans are treated as equal, we are all more free " he said.
However, Christian conservatives condemned the decision. Former Arkansas governor and presidential candidate Mike Huckabee called it "an out-of-control act of unconstitutional, judicial tyranny".
And Kellie Fiedorek, a lawyer for an anti-gay marriage advocacy group, said the decision "ignored the voices of thousands of Americans".
Greg Abbott, the governor of Texas, a state where marriages licences will now be issued to same-sex couples, said the justices "have imposed on the entire country their personal views on an issue that the Constitution and the Court's previous decisions reserve to the people of the states".
Loud cheers erupted outside the court after the ruling was announced, and there were tears, hugs, and cheers of "USA USA USA!".
A sea of rainbow flags overwhelmed the few anti-gay marriage activists who reacted in disbelief, and the demonstration seemed to turn into a street party.
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