Chesterfield is a city in St. Louis County, Missouri, United States. It is a western suburb of St. Louis. As of the 2020 census, the population was 49,999, making it the state's 14th most populous city. The broader valley of Chesterfield was originally referred to as "Gumbo Flats", derived from its soil, which though very rich and silty, resembled gumbo when wet.