The Ouachita River is a 605 mile long river that runs through the US states of Arkansas and Louisiana.

The Ouachita River begins in the Ouachita Mountains near Mena, Arkansas. The river flows generally southeast for 605 miles into Louisiana where it joins the Red River. The river has six locks and dams along its length. The lower portions of the river are also known as the Black River. Portions of the river flow through the Ouachita National Forest.

The river is named for the Washita Indian tribe. The Washita tribe was one of several tribes that lived along the river including the Caddo, Osage, Tensas, Chickasaw, and Choctaw. The word "Washita" is an Indian word meaning "Good hunting grounds" and "sparkling silver water". A mound building civilization existed along the river at one time as evidenced by the mounds found along its length. The largest such mound was destroyed during construction of a bridge at Jonesville, Louisiana in the 20th century. This mound was reported in use as late as 1540 by the explorer Hernando de Soto. A lightning strike destroyed the temple atop the mound that same year which was seen as a bad omen by the tribe. The temple was never rebuilt and the tribe moved away around 1730.

The river served as a route into northern Louisiana and southwestern Arkansas for European colonists and has served as an important transportation and trade route since the 18th century.

Near Hot Springs, Arkansas the Ouachita flows into a series of lakes including Lake Ouachita where it slows. The river joins the Tensas River and the Little River at Jonesville, Louisiana. The river is called the Black River from here until it joins the Red River.

Major towns along the river are: