Riverside is a town in Okanogan County, Washington, United States. The population was 348 at the 2000 census and decreased to 280 at the 2010 census.
Riverside was first settled in the 1880s by Uriah Ward. When steamboats began traveling up the Okanogan River, Riverside was the upstream limit of navigation during the high water season. Riverside was officially incorporated on December 22, 1913.
Riverside is located at 48°30′10″N 119°30′31″W / 48.50278°N 119.50861°W / 48.50278; -119.50861 (48.502830, -119.508718). According to the United States Census Bureau, the town has a total area of 1.0 square miles (2.5 km²), of which, 0.9 square miles (2.3 km²) of it is land and 0.1 square miles (0.2 km²) of it (8.25%) is water.