The contest has importance as it is being held three days before Super Tuesday, when voters in 10 states will either pick or begin to pick their presidential nominees. All four of the GOP's current candidates have campaigned extensively in the state.
Republican Party caucuses meet in their precinct locations starting at 10 a.m. PST. Only registered voters can caucus. The process is expected to finish by noon.
Participants will elect delegates to their county conventions, and they also will vote in a presidential straw poll by writing down their presidential preference.
Statewide results of the presidential straw poll will be released to the media Saturday evening, according to the Washington State Republican Party Caucus Web site.
Washington will have 40 bound delegates at the Republican National Convention in August, but the presidential straw poll Saturday has no bearing on the state's delegate-allocation process.