St. Charles County is in the central eastern part of the U.S. state of Missouri. As of the 2020 census, the population was 405,262, making it Missouri's third-most populous county. Its county seat is St. Charles. The county was organized October 1, 1812, and named for Saint Charles Borromeo, an Italian cardinal.

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.