Belleville is a city in St. Clair County, Illinois, United States. The population was 41,410 at the 2000 census. It is the county seat of St. Clair County, and the seat of the Roman Catholic Diocese of Belleville and the National Shrine of Our Lady of the Snows. Belleville, located within Southern Illinois, is included in the Metro-East region of the St. Louis Metropolitan Area.