Washington County is a county in the U.S. state of Idaho. As of the 2020 census, the population was 10,500. The county seat and largest city is Weiser, with over half of the county's population. The county was established in 1879 when Idaho was a territory and named after U.S. President George Washington.

Weiser is a city in the rural western part of the U.S. state of Idaho and the county seat of Washington County. With its mild climate, the city supports farm, orchard, and livestock endeavors in the vicinity. The city sits at the confluence of the Weiser River with the great Snake River, which marks the border with Oregon. The population was 5,507 at the 2010 census.