The measure now heads to the House, which is expected to approve it. Gov. Chris Gregoire supports the measure and has said she will sign it into law, though opponents have promised to challenge it at the ballot with a referendum.
The measure was passed on a 28-21 vote. Four Republicans crossed party lines and voted with majority Democrats for the measure. Three Democrats voted against it.
Any challenge to the bill can't be filed until after it is passed by the full Legislature and signed into law by Gregoire. Opponents then must turn in 120,577 signatures by June 6.
If opponents aren't able to collect enough signatures, gay and lesbian couples would be able to be wed starting in June. Otherwise, they would have to wait until the results of a November election.
Before last week, it wasn't certain the Senate would have the support to pass the measure, as a handful of Democrats remained undecided.
But after the first public hearing on the issue Jan. 23, a previously undecided Democratic senator, Mary Margaret Haugen of Camano Island, said she would be the 25th and deciding vote in support of the bill, all but ensuring its passage.
Same-sex marriage is legal in New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the District of Columbia.
Lawmakers in New Jersey and Maryland are expected to debate gay marriage this year, and Maine could see a gay marriage proposal on the November ballot.
The debate over same-sex marriage in Washington state has changed significantly since lawmakers passed Washington's Defense of Marriage Act in 1998, which banned gay marriage. The constitutionality of DOMA was ultimately upheld by the state Supreme Court in 2006, but earlier that year, a gay civil rights measure passed after nearly 30 years of failure.
The quick progression of domestic partnership laws in the state came soon after, with a domestic partnership law in 2007, and two years of expansion that culminated in 2009 with the so-called "everything but marriage law" that was upheld by voters after opponents filed a referendum to challenge it.
Under the measure that passed Wednesday, the more than 9,300 couples currently registered in domestic partnerships would have two years to either dissolve their relationship or get married. Domestic partnerships that aren't ended prior to June 30, 2014, would automatically become marriages.
Domestic partnerships would remain for senior couples where at least one partner is 62 years old or older. That provision was included to help seniors who don't remarry out of fear they could lose certain pension or Social Security benefits.
