The Bois de Sioux River (/ˌbɔɪz də ˈsuː/ BOYZ də SOO) drains Lake Traverse, the southernmost body of water in the Hudson Bay watershed of North America. It is a tributary of the Red River of the North and defines part of the western border of the U.S. state of Minnesota, and the eastern borders of North Dakota and South Dakota. It is about 41 miles (66 km) in length.

Bois de Sioux is a name derived from French meaning "Woods of the Sioux".

The river flows northward from a Corps of Engineers dam at the north end of Lake Traverse and shortly enters Mud Lake. Downstream of Mud Lake it is a small stream, and its flow has been channelized and straightened in some places so that the watercourse diverges slightly from the historical state boundary. It passes the town of White Rock, South Dakota, before joining the Otter Tail River to form the Red River of the North at Wahpeton, North Dakota, and Breckenridge, Minnesota.

The two largest tributaries of the Bois de Sioux are the Mustinka River, which flows into Lake Traverse, and the Rabbit River south of Breckenridge; both of these enter from Minnesota.

• List of rivers of Minnesota
• List of rivers of North Dakota
• List of rivers of South Dakota
• List of river borders of U.S. states

• Waters, Thomas F. (1977). The Streams and Rivers of Minnesota. Minneapolis: University of Minnesota Press. ISBN 0-8166-0960-8.