St. Clair County, Illinois

St. Clair County
County
St. Clair County Courthouse in Belleville
Flag Seal
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 / 38.47; -89.93
Country	United States
State	Illinois
Founded	1790
Named for	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
• Density	380/sq mi (150/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Congressional districts	12th, 13th
Website	www.co.st-clair.il.us

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’s seat and largest city.^[1]

Cahokia Village was founded in 1697 by French settlers and served as a Jesuit mission to convert tribes of the Illinois Confederation to Christianity. Prior to the establishment of Illinois as a state, the government of the Northwest Territory created St. Clair County in 1790 out of the western half of Knox County. 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 in St. Clair County.^[2]