St. Louis County is located in the eastern-central portion of Missouri. It is bounded by the City of St. Louis and the Mississippi River to the east, the Missouri River to the north, and the Meramec River to the south. At the 2020 census, the total population was 1,004,125, making it the most populous county in Missouri. Its county seat is Clayton. The county is included in the St. Louis, MO–IL metropolitan statistical area.
