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. Its county seat is Belleville. The county was founded in 1790, before the establishment of Illinois itself.
St. Clair County is part of the American Bottom or Metro-East area of the St. Louis, MO-IL Metropolitan Statistical Area.
In 1970, the U.S. Census Bureau placed the mean center of U.S. population in St. Clair County.