Gregoire signed the bill in the state reception room in the Capitol, surrounded by gay rights supporters. It's a historic moment, but same-sex couples can't walk down the aisle just yet.
The law takes effect June 7, but opponents are already mounting challenges on multiple fronts.
Opponents planned to file a challenge Monday that could put the law on hold pending the outcome of a November vote. Separately, an initiative was filed at the beginning of the session that opponents of gay marriage say could lead to the new law being overturned.
The Democratic governor signed the bill as Republican presidential candidate Rick Santorum, who opposes gay marriage, was in town speaking with conservative voters.
Santorum planned to meet with Republican lawmakers at the Capitol later Monday.
Gregoire's signature comes nearly a week after a federal appeals court declared California's ban on gay marriage unconstitutional, saying it was a violation of the civil rights of gay and lesbian couples.
A three-judge panel of the 9th Circuit Court of Appeals gave gay marriage opponents time to appeal the 2-1 decision against Proposition 8 before ordering the state to allow same-sex weddings to resume. The judges also said the decision only applies to California, even though the court has jurisdiction in nine Western states.
Washington state has had domestic partnership laws since 2007, and in 2009 passed an "everything but marriage" expansion of that law, which was ultimately upheld by voters after a referendum challenge.
Opponents said they would file a ballot challenge after Gregoire signed the measure that would require voters in November to either uphold or overturn the law. If the referendum gets enough signatures the law is put on hold pending the outcome of a November vote.
Separately, an anti-gay marriage initiative was filed at the beginning of the session, but the language is still being worked out so no signatures have been collected yet. An initiative alone would not pause the law.
Gay marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and Washington, D.C.
Same-sex marriage also has the backing of several prominent Pacific Northwest businesses, including Microsoft Corp., Nike Inc. and Starbucks Corp.
Legislative committees in Maryland heard testimony on gay marriage last week. Lawmakers in New Jersey are expected to vote on gay marriage soon, with a vote in the state Senate on Monday and a vote in the Assembly on Thursday.
N.J. Gov. Chris Christie, who is pushing for a public vote on the issue, says he'll veto the bill if it comes to his desk. Maine could see a gay marriage proposal on the November ballot.
Proposed amendments to ban gay marriage will be on the ballots in North Carolina in May and in Minnesota in November.
The gay marriage bill is Senate Bill 6239.