Belleville is a city in St. Clair County, Illinois, United States, co-extensive with Belleville Township. As estimated by the US Census Bureau in 2012, shown on the city's economic development page, the city has a population of 43,765 . It is the eighth-most populated city outside of the Chicago Metropolitan Area and the most populated city south of Springfield in the state of Illinois. 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 is the most populated city in the Metro-East region of the St. Louis Metropolitan Area and in Southern Illinois. Due to its close proximity to Scott Air Force Base, the population receives a boost from military and federal civilian personnel, defense contractors, and military retirees.