Grand River National Grassland is a National Grassland in northwestern South Dakota, United States. It is named for the Grand River. The North and South forks of the rivers meet in the grassland. It has a land area of 154,783 acres (62,638 ha). In descending order of acreage it lies in parts of Perkins, Corson, and Ziebach counties. The portion in Corson County also lies within the Standing Rock Indian Reservation. The portion in Ziebach County also lies within the Cheyenne River Indian Reservation.