St. Clair County is the oldest county in Illinois; its western border is formed by the Mississippi River, bordering Missouri. It is a part of the Metro East in southern Illinois. According to the 2020 United States Census, it had a population of 257,400, making it the eighth-most populous county in Illinois and the most populous in the southern portion of the state. Its county seat is Belleville. The county was founded in 1790 by the government of the Northwest Territory, from the western half of Knox County, before the establishment of Illinois as a state as the base for the Illinois Territory. Cahokia Village in the county was founded in 1697 and was a French settlement and former Jesuit mission.
Columbia is a city mainly in Monroe County with a small portion in St. Clair County in the U.S. state of Illinois, about 12 miles (19 km) south of St. Louis, Missouri. The population was 10,999 at the 2020 census.
As of the 2010 census, its population was 10,007 and it contained 4,394 housing units.
Canteen Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 10,263 and it contained 4,027 housing units. Canteen Township was formed from Centreville Station Township when it was subdivided on March 5, 1910.
Caseyville Township is located in St. Clair County, Illinois, United States. As of the 2010 census, its population was 31,996 and it contained 14,054 housing units.
Centreville Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 25,386 and it contained 10,535 housing units. Centreville Township was formed from Centreville Station when it was subdivided on March 5, 1910.
Engelmann Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 726 and it contained 288 housing units.
Fayetteville Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 1,668 and it contained 722 housing units.
Freeburg Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 5,109 and it contained 2,118 housing units.
Lebanon Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 4,538 and it contained 2,005 housing units.
Lenzburg Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 1,047 and it contained 450 housing units.
Marissa Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 2,468 and it contained 1,175 housing units.
Mascoutah Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 8,217 and it contained 3,335 housing units.
New Athens Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 2,657 and it contained 1,129 housing units.
O'Fallon Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 26,053 and it contained 10,338 housing units.
Shiloh Valley Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 11,631 and it contained 4,284 housing units. Shiloh Valley Township was originally called Shiloh before its name was changed on an unknown date.
Smithton Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 4,275 and it contained 1,684 housing units.
St. Clair Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 35,498 and it contained 15,366 housing units. St. Clair Township formed from a portion of Belleville Township on October 5, 1885.
Stites Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 749 and it contained 326 housing units. Stites Township was originally named Brooklyn Township when it was reorganized from a portion of East St. Louis Township in February, 1888. Its name was changed to Stites in April, 1888.
Sugarloaf Township is located in St. Clair County, Illinois. As of the 2010 census, its population was 7,322 and it contained 3,219 housing units.
United States 
Illinois 
St. Clair 
