The National Organization for Marriage claims that other Republicans vying for the presidential nomination such as Michele Bachmann have also signed the pledge.
Republican presidential contender Rick Perry has signed a pledge vowing to support a Constitutional amendment declaring marriage to be a union of one man and one woman, the group sponsoring the pledge said Friday.
"Kudos to Governor Rick Perry for making it clear: he's a marriage champion," National Organization for Marriage president Brian Brown said in a statement.
By signing the pledge, "Perry makes crystal clear that, contrary to the conventional wisdom, gay marriage is going to be even a bigger issue in 2012 than it was in 2008," Mr Brown said, "because the difference between the GOP nominee and President (Barack) Obama is going to be large and clear."
The pledge also includes a vow to support the Defense of Marriage Act in court, appoint judges and an Attorney General who will respect "the original meaning of the Constitution," and appoint a presidential commission to investigate harassment of "traditional marriage" supporters.
Michelle Bachmann, Mitt Romney and Rick Santorum, among those competing with Mr Perry for Republican presidential nomination, have also signed the pledge, the group said.
Mr Perry's move was immediately attacked by Democrats and supporters of gay marriage in his home state.
"I'm afraid that this kind of thing is going to be required by the electorate of the Republican Party to win their nomination," said Matt Glazer, executive director of Progress Texas, a lobbying group for liberal causes. "But this pledge will kill him with independent voters."
Mr Glazer said Mr Perry promised in his August 13 speech in South Carolina announcing his candidacy "to make Washington, D.C., as insignificant in our lives as possible".
"He says he wants to make sure that the federal government isn't involved in much, and then he promotes amazingly invasive federal government policies like this," Mr Glazer said.
Mr Perry had previously appeared to endorse the idea that under the Constitution it should be up to individual states to decide marriage issues.
He later backed away from that comment after criticism from Republican activists.
Mr Brown said his group's stance on marriage "is a winning position for a presidential candidate"