Chesterfield is a city in St. Louis County, Missouri, United States, and a Western suburb of St. Louis. As of the 2010 census, the population was 47,484, making it the state's fourteenth-largest city.The broader valley of Chesterfield was originally referred to as "Gumbo Flats", derived from its soil, which though very rich and silty, became like a gumbo when wet. Geographically, Chesterfield is half the size of St. Louis.