The unanimous ruling said supporters of Proposition 8 have legal standing to step because California's governor and attorney general declined to defend it in court against charges the ban on homosexual marriages is unconstitutional.
The ruling stated it was vital to the state's ballot initiative system for someone to be able to argue on behalf of a measure when state officials decline.
The ruling does not decide the constitutionality of the contentious issue; however, the Los Angeles Times said it sets the stage for a court battle likely to reach the U.S. Supreme Court. The issue has already reached the 9th U.S. Circuit Court of Appeals.
Opponents of Prop 8, which restricts marriage to heterosexuals, had argued only the state, not private citizens, had the legal standing to defend a state law like Prop 8 in court.
Prop 8 was passed by the voters in 2008 but a federal judge in San Francisco overturned it after a two-week trial.