High Quality Content by WIKIPEDIA articles! Lewiston is a city in and also the county seat of Nez Perce County in the Pacific Northwest state of Idaho. It is the second-largest city in the northern Idaho region, behind Coeur d`Alene and ninth-largest in the state. Lewiston is the principal city of the Lewiston, ID - Clarkston, WA Metropolitan Statistical Area, which includes all of Nez Perce County and Asotin County, Washington. As of the 2010 census, the population of Lewiston was 31,894.