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.

The Metro East is an urban area in Southern Illinois, United States that contains the eastern and northern suburbs and exurbs of St. Louis, Missouri. It encompasses five counties in the Greater St. Louis area and constitutes the second-most populous urban area in Illinois. The region's most populated city is Belleville, with 42,404 residents.