Those who want the state law overturned gathered 122,481 signatures, more than twice the number needed to get the issue on the ballot, The Baltimore Sun reported.
If at least 55,736 of those signatures are certified as valid, the referendum will be held in November.
If voters support allowing gay couples to marry, it will become legal in January 2013.
The date was chosen deliberately when the state Legislature approved a same-sex marriage bill to allow time for a vote.
The Rev. Derek McCoy, head of the Maryland Marriage Alliance, said opponents of same-sex marriage have been roused by recent endorsements of it from President Obama and the National Association for the Advancement of Colored People.
"Every day we have had more and more momentum," he said.
The Rev. MacArthur Flournoy, faith leader for Marylanders for Marriage Equality, said surveys done for the group show public opinion is swinging toward gay marriage.
"We expected them to make their numbers," he said. "Once stories are told, people see that hearts and minds are changed."
So far, gay marriage has failed in every state where the issue has been on the ballot.