Flandreau /ˈflændruː/ FLAN-droo is a city in Moody County, South Dakota, United States. The population was 2,341 at the 2010 census. It is the county seat of Moody County. It was named in honor of Charles Eugene Flandrau, a judge in the territory and state of Minnesota, who is credited with saving the community of New Ulm, Minnesota, from destruction during conflict with the Sioux in 1862.