Belleville is a city in St. Clair County, Illinois, United States, co-extensive with Belleville Township. As of the 2010 census, the city has a population of 44,478. 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.