Belvidere is a city in Boone County, on the northern border of Illinois, United States. The population was 25,339 as of the 2020 census. It is the county seat of Boone County. Belvidere is part of the Rockfo…Belvidere is a city in Boone County, on the northern border of Illinois, United States. The population was 25,339 as of the 2020 census. It is the county seat of Boone County. Belvidere is part of the Rockford, Illinois Metropolitan Statistical Area.