Janesville is a city in southern Wisconsin, United States. It is the county seat of Rock County and the principal municipality of the Janesville, Wisconsin Metropolitan Statistical Area. As of the 2000 census, the city had a population of 59,498.
Janesville was founded in 1835 on the east bank of the Rock River. Named for an early settler, Henry Janes, early development depended on water power from the river and an early territorial road that included the first bridge across the Rock River.
Janesville was the site of the first Wisconsin State Fair in 1851, attended by approximately 10,000 people.