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.

The Rock River is a tributary of the Mississippi River, approximately 299 miles (481 km) long, in the U.S. states of Wisconsin and Illinois. The river was known as the Sinnissippi to Sauk and Meskwaki peoples; the name means "rocky waters".