Saint Charles is a city in, and the county seat of, St. Charles County, Missouri, United States. The population was 70,493 at the 2020 census, making St. Charles the 9th most populous city in Missouri. Situated on the Missouri River, St. Charles is a northwestern suburb of St. Louis. The city was founded circa 1769 as Les Petites Côtes, or "The Little Hills" in French, by Louis Blanchette, a French-Canadian fur trader.