Flandreau FLAN-droo is a city in and county seat of Moody County, South Dakota, United States. The population was 2,372 at the 2020 census. It was named in honor of Charles Eugene Flandrau, a judge in the territory and state of Minnesota. He is credited with saving the community of New Ulm, Minnesota, from destruction during conflict with the Sioux tribe in 1862.

Charles Eugene Flandrau was an American lawyer who became influential in the Minnesota Territory, and later state, after moving there in 1853 from New York City. He served on the Minnesota Territorial Council, in the Minnesota Constitutional Convention, and on the Minnesota territorial and state supreme courts. He was also an associate justice on the Minnesota Supreme Court.