Rockford is a city in Winnebago and Ogle counties in the U.S. state of Illinois. It is located in far northern Illinois on the banks of the Rock River. The fifth-most populous city in Illinois with a population of 148,655 at the 2020 census, it anchors the Rockford metropolitan area with 337,000 residents. Rockford is the county seat of Winnebago County and the most populous city in Illinois outside the Chicago metropolitan area.