St. Charles is a city in Madison County, Iowa, United States. The population was 640 at the time of the 2020 census. It is part of the D…St. Charles is a city in Madison County, Iowa, United States. The population was 640 at the time of the 2020 census. It is part of the Des Moines–West Des Moines Metropolitan Statistical Area. St. Charles was platted in 1852. It was named after the city of St. Charles, Missouri. St. Charles was incorporated as a city in 1876.