The ruling, in the case of Obergefell v. Hodges, paves the way for gay marriage to become the law of the land in all 50 states, just 12 years after Massachusetts became the first state to permit same-sex unions.
Gay marriage is already legal in 37 states and the Washington, D.C., and popular support for it has grown rapidly over the past decade.
Friday’s ruling means the remaining 13 states where same-sex unions haven’t previously been permitted cannot deny same-sex couples the right to marry.
President Obama, who opposed gay marriage as a candidate for president in 2008, reversed his position and said in 2012 that he had evolved into a gay marriage supporter. He was the first president or presidential candidate from either party to support the cause.
The ruling is the second of historic significance for the high court, which upheld Obamacare in a 6-3 ruling on Thursday.
Read the court’s full gay marriage ruling here: