Buffalo County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 1,948. Its county seat is Gann Valley which, at 10 people, is the least populous county seat in the United States. The county was created in 1864, and was organized in 1871 as part of the Dakota Territory. In 2010, the center of population of South Dakota was located in eastern Buffalo County.