The Big Sioux River is a tributary of the Missouri River in eastern South Dakota and northwestern Iowa in the United States. It flows generally southwardly for 419 mi (674 km), and its watershed is 9,006 sq mi (23,330 km). The United States Board on Geographic Names settled on "Big Sioux River" as the stream's name in 1931. The river was named after the Lakota people which was known by them as Tehankasandata, or Thick Wooded River.
The Big Sioux River rises in Roberts County, South Dakota, on a low plateau known as the Coteau des Prairies and flows generally southwardly through Grant, Codington, Hamlin, Brookings, Moody, and Minnehaha counties, past the communities of Watertown, Castlewood, Bruce, Flandreau, Egan, Trent, Dell Rapids, and Baltic to Sioux Falls, where it passes over a waterfall in Falls Park, which gives that city its name. Downstream of Sioux Falls and the community of Brandon, the Big Sioux defines the boundary between South Dakota and Iowa, flowing along the eastern borders of Lincoln and Union counties in South Dakota, and the western borders of Lyon, Sioux and Plymouth counties in Iowa, past the communities of Canton, Fairview, Hudson, Hawarden, North Sioux City, and Dakota Dunes in South Dakota and Beloit, Hawarden and Akron in Iowa. It joins the Missouri River from the north at Sioux City, Iowa.