Kennedy, a conservative who often casts the deciding vote in close cases, was joined in the majority by the court’s four liberal justices.
In a dissenting opinion, conservative Justice Antonin Scalia said the decision shows the court is a “threat to American democracy.” The ruling “says that my ruler and the ruler of 320 million Americans coast-to-coast is a majority of the nine lawyers on the Supreme Court,” Scalia said.
Conservative Chief Justice John Roberts also read a summary of his dissenting opinion from the bench.
There are currently 13 state bans in place, while another state, Alabama, has contested a court ruling that lifted the ban there.
The ruling is the Supreme Court’s most important expansion of marriage rights in the United States since its landmark 1967 ruling in the case Loving v. Virginia that struck down state laws barring interracial marriages.
The ruling is the latest milestone in the gay rights movement in recent years. In 2010, Obama signed a law allowing gays to serve openly in the U.S. military. In 2013, the high court ruled unconstitutional a 1996 U.S. law that declared for the purposes of federal benefits marriage was defined as between one man and one woman.
Read More: Supreme Court rules in favor of gay marriage nationwide | Reuters