Belleville
- County:
- St. Clair County
- County Seat:
- Yes
- Area (mi²):
- 23.233
- State:
- Illinois
Belleville is a city located in St. Clair County, Illinois. Belleville has a 2026 population of 40,560. It is also the county seat of St. Clair County. Belleville is currently declining at a rate of -0.77% annually and its population has decreased by -4.06% since the most recent census, which recorded a population of 42,278 in 2020.