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, making it the ninth-most populous county in Illinois. Its county seat is Belleville. The county was founded in 1790, before the establishment of Illinois as a state.

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Category:1790s establishments in the Northwest Territory
Category:1790 establishments in the United States
Category:Illinois counties