Rock County is a county in the U.S. state of Wisconsin. As of the 2020 census, the population was 163,687. Its county seat is Janesville. Rock County comprises the Janesville-Beloit, WI Metropolitan Statistical Area and is included in the Madison-Janesville-Beloit, WI Combined Statistical Area.

Janesville is the most populous city and the county seat of Rock County, Wisconsin, United States. It is a principal municipality of the Janesville, Wisconsin, Metropolitan Statistical Area and is included in the Madison–Janesville–Beloit, WI Combined Statistical Area. As of the 2020 census, the city had a population of 65,615, making it the tenth-most populous city in Wisconsin.