Bennett County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 3,381. Its county seat is Martin. The county lies completely within the exterior boundary of the Pine Ridge Indian Reservation. To the east is the Rosebud Indian Reservation, occupied by Sicangu Oyate, also known the Upper Brulé Sioux Nation and the Rosebud Sioux Tribe (RST), a branch of the Lakota people.