Onalaska is a city in La Crosse County, Wisconsin. It is part of the La Crosse Metropolitan Area. Onalaska is built on a slightly elevated ridge above the Black River. Natural areas include river bottomland as well as high, heavily wooded, scenic bluffs. A manmade reservoir at the city's western edge is known as Lake Onalaska. Onalaska is known as "The Sunfish Capital of the World."