The court rejected arguments by ProtectMarriage, sponsors of Proposition 8, that only an appellate court has the authority to overturn a state law, the Los Angeles Times reported.
California voters approved Proposition 8 in 2008, after the state Supreme Court found laws banning same-sex marriage violated constitutional guarantees of equal protection.
A federal judge in 2010 found Proposition 8 itself was unconstitutional and -- after state officials declined to appeal the ruling to the U.S. Supreme Court -- ProtectMarriage took the case to the nation's highest court, which ruled in June the initiative's sponsors lacked standing to defend the measure in federal court.
The decision left in place the ruling by retired Chief U.S. District Judge Vaughn R. Walker.
ProtectMarriage then appealed to the California Supreme Court, arguing -- among other things -- that the federal judge who overturned Proposition 8 lacked authority to do so.