The Ouachita Mountains (/ˈwɒʃɪtɔː/), simply referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of highly deformed Paleozoic strata constituting the Ouachita Fold and Thrust Belt, one of the important orogenic belts of North America. The Ouachitas continue in the subsurface to the northeast, where they make a poorly understood connection with the Appalachians and to the southwest, where they join with the Marathon uplift area of West Texas. Together with the Ozark Plateaus, the Ouachitas form the U.S. Interior Highlands. The highest natural point is Mount Magazine at 2,753 feet (839 m).

The Ouachita Mountains is a Level III ecoregion designated by the Environmental Protection Agency (EPA). The region has been subdivided into six Level IV ecoregions.

Louis R. Harlan claimed that "Ouachita" is composed of the Choctaw words ouac for "bison" and chito for "large", together meaning "country of large bison". At one time, herds of bison inhabited the lowland areas of the Ouachitas. Historian Muriel H. Wright wrote that "Ouachita" is composed of the Choctaw words owa for "hunt" and chito for "big", together meaning "big hunt far from home". According to the article Ouachita in the Encyclopedia of Oklahoma History and Culture, "Ouachita" comes from the French spelling of the Caddo word washita, meaning "good hunting grounds".

The Ouachitas are a major physiographic province of Arkansas and Oklahoma and are generally grouped with the Arkansas River Valley. Together with the Ozark Plateaus, the Ouachitas form the U.S. Interior Highlands, one of few mountainous regions between the Appalachians and Rockies.

The Ouachitas are dominated by pine, oak, and hickory. The shortleaf pine (Pinus echinata) and post oak (Quercus stellata) thrive in dry, nutrient-poor soil and are common in upland areas. The maple-leaf oak (Quercus acerifolia) is found at only four sites worldwide, all of which are in the Ouachitas. Some native tree species, such as the eastern red-cedar (Juniperus virginiana), are colonizers of human-disturbed sites.

The Ouachita National Forest covers approximately 1.8 million acres of the Ouachitas. It is one of the largest and oldest national forests in the Southern U.S., created through an executive order by President Theodore Roosevelt on December 18, 1907. There are six wilderness areas within the Ouachita National Forest, which are protected areas designed to minimize the impacts of human activities.

Several plants are endemic to the Ouachitas, including: Agalinis nuttallii, Cardamine angustata var. ouachitana, Carex latebracteata, Galium arkansanum var. pubifolium, Houstonia ouachitana, Hydrophyllum brownei, Liatris compacta, Quercus acerifolia, Polymnia cossatotensis, Solidago ouachitensis, Spiranthes niklasii (near endemic, also found on Crowley's Ridge), Streptanthus maculatus subsp. obtusifolius, Streptanthus squamiformis, Valerianella palmerii, and Vernonia lettermanii.

Bison and elk once found habitat in the Ouachita Mountains, but have since been extirpated. Today, there are large populations of white-tailed deer, coyote, and other common temperate forest animals. Though elusive, hundreds of black bear roam the Ouachitas. Several cryptic species of salamander are endemic to the Ouachitas and have traits that vary from one locale to another.