Custer County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 8,318. Its county seat is Custer. The county was created in 1875, and was organized in 1877. It was named after General George Armstrong Custer. Custer County is home to two of the three longest caves in the United States: Jewel Cave National Monument and Wind Cave National Park.