Rockford is the third largest city in the U.S. state of Illinois, the 171st most populous city in the United States, and the 148th most populous region in the United States. It is the county seat of Winnebago County and is located on both banks of the Rock River in far northern Illinois. According to 2010 census data, the City of Rockford had a population of 152,871, with an outlying metropolitan area population of 348,360, which was estimated to have decreased to 150,251 and 344,623, respectively, by July 2013.