Washington became the seventh state in the US to legalise same-sex marriage after Democrat governor Christine Gregoire signed the bill into law. It was approved despite opposition from conservative legislators who vowed to overturn it through a referendum.
Opponents have launched a campaign to collect enough signatures to force the referendum, which could be used as a delaying tactic.
If they fail to collect enough signatures to force a referendum, the law will take effect on 7 June.
The signing of the bill in to law is seen as landmark victory for the LGBT rights activists, who are campaigning in the country for equal rights for the community.
Gregoire, a devout catholic, said after the signing of the bill that she is proud of the state as it stands for equality. I ask all Washingtonians to look into your hearts and ask yourselves -- isn't it time? ... We in this state stand proud for equality, she said according to a Reuters report.
Six other U.S. states - Iowa, New Hampshire, Connecticut, Massachusetts, New York and Vermont along with the District of Columbia have already legalised same-sex marriage. The gay rights groups are pushing for similar legislation in other states like Maryland and New Jersey which has Democrat majority in the Senate.
While in Maine, a referendum will be held during the November polls to decide on the issue.
Same-sex marriage has been a hot issue in the US with religious and conservative groups vehemently opposing the move. The issue will also figure in the state's presidential poll primary campaign, as the state GOP caucus is scheduled on March 3. The GOP candidates, especially the ultra-conservative Rick Santorum and neo-ultra-conservative Mitt Romney, have criticised same-sex marriages in their campaign events.