Supporters of gay marriage said that although they were ready for a campaign battle, they were allowing themselves to celebrate first.
"You have to relish this moment," said Bret Tiderman, 31, of Seattle.
The state reception room at the Capitol was packed with hundreds of gay-rights supporters and at least 40 state lawmakers.
Sen. Ed Murray, a Seattle Democrat who is gay and who has sponsored gay-rights legislation for years, told the cheering crowd: "My friends, welcome to the other side of the rainbow. No matter what the future holds, nothing will take this moment in history away from us."
As the Democratic governor signed the bill, a man shouted, "Do not betray Christ!" His voice was overwhelmed by gay-marriage supporters who cheered and spoke loudly during his outburst. Bob Struble, 68, of Bremerton, was removed from the room, and he said security guards had given him a warning.
Struble said he believed the state would halt same-sex marriage in a public vote.
"We'll be doing everything we can to overturn this unfortunate law," he said.
Republican presidential candidate Rick Santorum, who opposes same-sex marriage, was in town speaking with conservative voters. Santorum also met with Republican lawmakers at the Capitol later Monday. The former Pennsylvania senator said he encouraged gay-marriage opponents "to continue the fight."
"There are ebbs and flows in every battle, and this is not the final word," he said.
Washington state has had domestic-partnership laws since 2007, and in 2009, it passed an "everything but marriage" expansion of that law, which voters ultimately upheld.
The coalition of opponents that filed Monday's referendum is called Preserve Marriage Washington. The National Organization for Marriage, which was involved in ballot measures that overturned gay marriage in California and Maine, has promised to work with Preserve Marriage Washington to qualify a referendum to overturn the new law.
Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont, and Washington, D.C. It also has the backing of several prominent Pacific Northwest businesses, including Microsoft Corp., Nike Inc., and Starbucks Corp.