RCW 26.16.180: Spouses or domestic partners may sue each other.

Should either spouse or either domestic partner obtain possession or control of property belonging to the other, either before or after marriage or before or after entering into a state registered domestic partnership, the owner of the property may maintain an action therefor, or for any right growing out of the same, in the same manner and to the same extent as if they were unmarried or were not in a state registered domestic partnership.