The Fort Thompson Mounds are a complex of ancient archaeological sites in Buffalo County, South Dakota, near Fort Thompson and within the Crow Creek Reservation. Declared a National Historic Landmark in 1964 by the US Department of Interior, the mound complex extends for a distance of about 6 miles (9.7 km) along the east bank of the Missouri River. It is one of the largest known complex of burial mounds in the Plains region north of Kansas.