St. Louis County is a county located in the U.S. state of Missouri. Its county seat is Clayton. St. Louis County is part of the St. Louis Metro Area wherein the independent city of St. Louis and its suburbs in St. Louis County, as well as the surrounding counties in both Missouri and Illinois all together account for a total population of almost 2.8 million people. St. Louis County borders the City of St. Louis, which is independent from St. Louis County. As of the 2010 census, the population was 998,954, making it the most populous county in Missouri and more than three times as populous as its adjoining namesake city.