St. Clair County, Illinois
St. Clair County | |
|---|---|
St. Clair County Courthouse in Belleville | |
Location within the U.S. state of Illinois | |
Illinois's location within the U.S. | |
| Coordinates: 38°28′N 89°56′W / 38.47°N 89.93°W | |
| Country | United States |
| State | Illinois |
| Founded | 1790 |
| Named after | Arthur St. Clair |
| Seat | Belleville |
| Largest city | Belleville |
| Area | |
• Total | 674 sq mi (1,750 km2) |
| • Land | 658 sq mi (1,700 km2) |
| • Water | 16 sq mi (40 km2) 2.4% |
| Population (2020) | |
• Total | 257,400 |
• Estimate (2023) | 251,018 |
| • Density | 380/sq mi (150/km2) |
| Time zone | UTC−6 (Central) |
| • Summer (DST) | UTC−5 (CDT) |
| Congressional districts | 12th, 13th |
| Website | www |
St. Clair County is the ninth most populous county in Illinois. Located directly east of St. Louis, the county is part of the Metro East region of the Greater St. Louis metropolitan area in southern Illinois. As of the 2020 United States census, St. Clair County had a population of 257,400, making it the third most populous county in Illinois outside the Chicago metropolitan area. Belleville is the county seat and largest city.
Along the Mississippi River, Cahokia Village was founded in 1697 by French settlers and served as a Jesuit mission to convert tribes of the Illinois Confederation to Christianity. The area became the center of the French Illinois Country. Prior to the establishment of Illinois as a state, the government of the Northwest Territory created St. Clair County in 1790. In 1809, the county became the administrative center of the Illinois Territory and one of the two original counties of Illinois, alongside Randolph County. In 1970, the United States Census Bureau placed the mean center of U.S. population, which generally has moved west every decennial census in St. Clair County.