- Lewiston is a city and the county seat of Nez Perce County, Idaho, United States, in the state’s north central region. It is the third-largest city in the northern Idaho region, behind Post Falls and Coeur d’Alene, and the twelfth-largest in the state. Lewiston is the principal city of the Lewiston, ID-WA Metropolitan Statistical Area, which includes all of Nez Perce County and Asotin County, Washington. As of the 2020 census, the population of Lewiston was 34,203, up from 31,894 in 2010.