Asotin County is a county located in Washington . It is part of the Lewiston, ID-WA Metropolitan Statistical Area, which includes all of both Nez Perce County, Idaho, and Asotin County. As of 2000, the population was 20,551. The county seat is at Asotin, and its largest city is Clarkston. "Asotin" derives from a Nez Perce word meaning "Eel Creek". Asotin County was created out of Garfield County on October 27, 1883.