Asotin County, Washington (Wikipedia) Asotin County (/əˈsoʊtən/) is a county in the far southeastern corner of the U.S. state of Washington. As of the 2020 census, the population was 22,285. The county seat is at Asotin, and its largest city is Clarkston. The county was created out of Garfield County in 1883 and derives its name from a Nez Perce word meaning “Eel Creek.” Asotin County is part of the Lewiston–Clarkston metropolitan area, which includes Nez Perce County, Idaho, and Asotin County.