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. Bennett County was removed from the Pine Ridge Reservation by an act of Congress in 1911, much of Bennett County however is still held in trust by the Reservation. To the east is the Rosebud Indian Reservation, whe…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. Bennett County was removed from the Pine Ridge Reservation by an act of Congress in 1911, much of Bennett County however is still held in trust by the Reservation. To the east is the Rosebud Indian Reservation, where dwell the Sicangu Oyate, also known the Upper Brulé Sioux Nation and the Rosebud Sioux Tribe, a branch of the Lakota people.