Boone County is a county located in the U.S. state of Illinois. As of the 2020 census, the population was 53,448. Its county seat is Belvidere.

Belvidere is a city and the county seat of Boone County, Illinois. It is settled on the Kishwaukee River in far northern Illinois, United States. Known as the 'City of Murals', Belvidere is home to several public art installations throughout the North and South State Street historic districts, which are on the National Register of Historic Places. These historic districts are home to places like the Boone County Museum of History, The Funderburg Museum, several restaurants and bars, antique stores and Boutique stores. Popular festivities like Heritage Days are held yearly. The population was 25,339 as of the 2020 census. Belvidere is part of the Rockford, Illinois Metropolitan Statistical Area.