Download coordinates as: 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.

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.