Janesville is a city in Black Hawk and Bremer counties in the U.S. state of Iowa. The population was 1034 at the 2020 census. It is part of the Waterloo–Cedar Falls Metropolitan Statistical Area.

Black Hawk County is a county in the northeastern part of U.S. state of Iowa. As of the 2020 census, the population was 131,144, making it Iowa's fifth-most populous county. The county seat is Waterloo.