California Secretary of State Debra Bowen said Monday that a random check of signatures submitted by the measure's sponsors showed that they had gathered enough names for it to be put to voters.
The measure would amend the state constitution to "provide that only marriage between a man and a woman is valid or recognized in California."
If approved by a majority of voters on Nov. 4, the amendment would overturn the recent California Supreme Court ruling that legalized same-sex marriage in the state. It is similar to gay marriage bans that have been adopted in 26 other states.
"This signifies the fact that California voters really do favor and will come out to vote for the protection of historic marriage," said Ron Prentice, executive director of ProtectMarriage.com, a coalition of religious and social conservative groups behind the initiative.
In response to the court's May 15 ruling, California public health officials already have amended marriage license applications to read "Party A" and "Party B" instead of bride and groom. Local officials have been told to start issuing the revised licenses to same-sex couples on June 17.
Gay men and lesbians would still be able to get married between then and the election, even with the initiative pending, unless the court agrees to stay its decision until after Nov. 4, as the amendment's sponsors have requested.
If the marriages proceed during the next five months, it is unclear whether they would be nullified if the amendment passes. Some legal scholars have said the state Supreme Court might get called on again to settle that question.
Kate Kendell, executive director of the San Francisco-based National Center for Lesbian Rights, said gay marriage advocates have already launched a campaign to defeat the measure.
"There is just so much at stake, now, in terms of what kind of state we are going to live in and what values we are going to uphold," Kendell said.
To qualify for the ballot, the measure needed 694,354 petition signatures, an amount equal to 8 percent of the votes cast during the last governor's race.
Proponents submitted 1,120,801 signatures in late April, and county clerks determined the measure qualified by verifying the validity of 3 percent of the signatures they received, according to Bowen.
Recent polls have found California voters are about evenly split on whether gay couples should be allowed to marry.