Rockford is a city in Winnebago County, Illinois, United States. Located in far northern Illinois on the banks of the Rock River, Rockford is the county seat of Winnebago County. The population was 148,655 at the 2020 census, making Rockford the fifth-most populous city in Illinois, as well as the state's most populous city outside of the Chicago metropolitan area. It anchors the Rockford metropolitan area, which had 338,798 residents in 2020.