Belvidere pronounced /ˈbɛlvɨˌdɪər/ 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 is part of the Rockford, Illinois Metropolitan Statistical Area.