The court's 5-4 vote Wednesday leaves in place the initial trial court declaration that the ban is unconstitutional. California officials probably will rely on that ruling to allow the resumption of same-sex unions in about a month's time.
The high court itself said nothing about the validity of gay marriage bans in California and roughly three dozen other states.
The outcome was not along ideological lines.
Chief Justice John Roberts wrote the majority opinion, joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Elena Kagan and Antonin Scalia.
"We have no authority to decide this case on the merits, and neither did the 9th Circuit," Roberts said, referring to the federal appeals court that also struck down Proposition 8.
Four justices, Anthony Kennedy, Samuel Alito, Clarence Thomas and Sonia Sotomayor, said the court should have decided the constitutional question that was before it.