St. Charles is a city in Winona County, Minnesota, United States. The population was 3,735 at the 2010 census. It promotes itself as the gateway to Whitewater State Park, which is located 7 miles (11 km) north of the city on Minnesota State Highway 74. The annual city festival is Gladiolus Days which is always held the weekend before labor day in August. It honors the late Carl H. Fischer, a local resident who developed many varieties of that flower.