Sioux County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 35,872. Its county seat is Orange City. Its largest city is Sioux Center.

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 km2). 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.