South Dakota Highway 391 (SD 391) is a 3.362-mile-long (5.411 km) state highway in the southwestern part of the U.S. state of South Dakota.

The highway is located entirely within Oglala Lakota County and in the Pine Ridge Indian Reservation.

[2] It is maintained by the South Dakota Department of Transportation (SDDOT), and is not part of the National Highway System.

SD 391 begins at the Nebraska state line as a continuation of N-27 and travels northeast through rolling plains on the Pine Ridge Indian Reservation in Oglala Lakota County.

The highway curves to the northwest, then bends to the north near its northern terminus.