St. Clair County, Illinois
St. Clair County
St. Clair County Courthouse, Belleville
Location within the U.S. state of Illinois
Illinois's location within the U.S.
|Named for||Arthur St. Clair|
|• Total||674 sq mi (1,750 km2)|
|• Land||658 sq mi (1,700 km2)|
|• Water||16 sq mi (40 km2) 2.4%%|
|• Density||411/sq mi (159/km2)|
|Time zone||UTC−6 (Central)|
|• Summer (DST)||UTC−5 (CDT)|
St. Clair County is the oldest county in the U.S. state of Illinois. According to the 2010 census, it had a population of 270,056, making it the eighth-most populous county in Illinois and the most populous in the southern part of the state. Its county seat is Belleville. The county was founded in 1790, before the establishment of Illinois as a state.