Belvidere is a city in Boone County, Illinois, United States. The population was 20,820 at the 2000 census. It is the county seat of Boone County. 2005 population estimates show a large increase, as the population rose to 24,593. Belvidere apartments for rent comparison page shows most up to date rent information on Belvidere apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.