The Moreau River is a tributary of the Missouri River, approximately 200 mi (320 km) long, in South Dakota in the United States. It rises in two forks in northwestern South Dakota, in the Badlands of Harding County.
The North Fork rises approximately 10 mi (16 km) northeast of Crow Buttes. The South Fork rises approximately 20 mi (32 km) west of the headwaters of the North Fork. The two forks flow ESE and unite near Zeona in southern Perkins County. The combined stream flows east, past Usta, and across the Cheyenne River Indian Reservation, past Iron Lightning, Thunder Butte, Green Grass and Whitehorse. It joins the Missouri in Lake Oahe, with the lower 25 mi (40 km) of the river forming an arm of the reservoir.
The Moreau flows across the Cheyenne River Indian Reservation, past Iron Lightning, Thunder Butte, Green Grass and Whitehorse. The Cheyenne Indian Reservation was created by the United States in 1889 by breaking up the Great Sioux Reservation, following its victory over the Lakota in a series of wars in the 1870s. The reservation covers almost all of Dewey and Ziebach counties in South Dakota.