Belleville is a city and the county seat of St. Clair County, Illinois, United States. The population was 42,404 at the 2020 census, making it the most populated city in the Metro East region of Greater St. Louis, and in all of Southern Illinois. Due to its proximity to Scott Air Force Base, the city has a significant population of military and federal civilian personnel. It is the seat of the Roman Catholic Diocese of Belleville and home to the National Shrine of Our Lady of the Snows.

St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the third most populous county in Illinois outside the Chicago metropolitan area. Belleville is the county’s seat and largest city.