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 Southern Illinois and the Metro East region of Greater St. Louis. 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.