St. Clair County, Illinois

St. Clair County is the oldest county in the U.S. state of Illinois. According to the 2020 census, it had a population of 257,400,^[1] making it the ninth-most populous county in Illinois. Its county seat is Belleville.^[2] The county was founded in 1790, before the establishment of Illinois as a state.

St. Clair County
County
St. Clair County Courthouse in Belleville
Seal
Location within the U.S. state of Illinois
Illinois's location within the U.S.
Coordinates: 38°28′N 89°56′W
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)[1]
• Total	257,400
• Density	380/sq mi (150/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Website	www.co.st-clair.il.us