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.
According to the 2010 census, the township has a total area of , of which (or 98.29%) is land and (or 1.71%) is water.