Gregoire signed House Bill 1649 Tuesday, which will take effect 90 days after the 2011 Legislature adjourns, Seattlepi.com reported.
"It really is quite significant," Gregoire said.
Five U.S. states, the District of Columbia and Canada currently allow same-sex marriages.
Washington becomes the fourth state to approve a bill regarding the rights of same-sex domestic partnerships, following Maryland, Rhode Island and New York.
Rights and responsibilities given to gay and lesbian families under the latest legislation include workers' compensation coverage, the right to use sick leave to care for a spouse, victims' rights including the right to receive notifications, and benefits allowances and business succession rights, Seattlepi.com reported.