The Walnut River is a tributary of the Arkansas River, 154 miles (248 km) long, in the Flint Hills region of Kansas in the United States. Via the Arkansas, it is part of the Mississippi River watershed.

The Arkansas River is a major tributary of the Mississippi River. It generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. The river's source basin lies in Colorado, specifically the Arkansas River Valley. The headwaters derive from the snowpack in the Sawatch and Mosquito mountain ranges. It flows east into Kansas and finally through Oklahoma and Arkansas, where it meets the Mississippi River.