The Big Sioux River is a tributary of the Missouri River in the upper Midwest of the United States.

Its headwaters are in Pelican Lake in northeastern South Dakota, near the town of Watertown in the Coteau des Prairies.

It flows roughly due south, through extreme eastern South Dakota and passes through the city of Sioux Falls. South of Sioux Falls, it forms part of the boundary between South Dakota and Iowa, before flowing into the Missouri north of the city of Sioux City, Iowa.

See also

• List of Iowa rivers
• List of South Dakota rivers