Rockford is a city in Winnebago County, Illinois, located in the far northern part of the state. Situated on the banks of the Rock River, Rockford is the county seat of Winnebago County. The largest city in Illinois outside of the Chicago metropolitan area, Rockford is the fourth-largest city in the state and the 171st most populous in the United States. According to 2010 U.S. Census data, the City of Rockford had a population of 152,871, with an outlying metropolitan area population of 348,360.