Janesville is a city in, and the county seat of Rock County, Wisconsin, United States. The population was 65,615 at the 2020 census, and was estimated at 66,428 in 2024, making it the tenth-most populous city in Wisconsin. The Janesville–Beloit Metropolitan statistical area, consisting solely of Rock County, has an estimated 165,461 residents in the 2024 estimate. It is located along the Rock River in southern Wisconsin.