Asotin County is a county located in the U.S. state of Washington. As of the 2010 census, the population was 21,623. The county seat is at Asotin, and its largest city is Clarkston. The county was created out of Garfield County on October 27, 1883 and derives its name from a Nez Perce word meaning "Eel Creek".Asotin County is part of the Lewiston, ID-WA Metropolitan Statistical Area, which includes all of both Nez Perce County, Idaho, and Asotin County.