The Canadian River is the longest tributary of the Arkansas River in the United States. It is about 1,026 miles (1,651 km) long, starting in Colorado and traveling through New Mexico, the Texas Panhandle, and Oklahoma. The drainage area is about 47,700 square miles (124,000 km²).

The Canadian is sometimes referred to as the South Canadian River to differentiate it from the North Canadian River that flows into it.

On John C. Fremont's route map of 1845, the river's name is listed as "Goo-al-pah or Canadian River" from the Comanche and Kiowa name for the river (Kiowa gúlvàu, [ɡúᵈl.pʼɔː] red river). In 1929, Muriel H. Wright wrote that the Canadian River was named about 1820 by French traders who noted another group of traders from Canada (Canadiens) had camped on the river near its confluence with the Arkansas River.

According to the Encyclopedia of Oklahoma History and Culture, Spanish explorers in the 17th and 18th centuries called it the Rio Buenaventura and the Magdalena. The upper part was called Rio Colorado by the Spanish.

A more recent explanation comes from William Bright, who wrote that the name is "probably derived from Río Canadiano", a Spanish spelling of the Caddo word káyántinu, which was the Caddo people's name for the nearby Red River.

The name could be of Spanish origin from the word cañada (meaning "glen"), as the Canadian River formed a steep canyon in northern New Mexico and a somewhat broad canyon in Texas. A few historical records document that explanation. Edward Hale, writing in 1929, considered the French origin of the name most probable. In fact, the river was regularly used by Canadian fur traders (such as Louis Feuilli and Jean Chapuis, as well as the Mallet brothers) trying to establish contact with Santa Fe as early as 1752.

The first European to explore the Canadian River was Juan de Oñate, the Spanish governor of New Mexico, who followed the river from its origin to the western plains of what is now Oklahoma in 1601. Spanish traders and hunters were soon actively working in this area. French voyageurs were active along the lower Canadian. Bénard de la Harpe explored between the mouth of the river and the Kiamichi Mountains in 1715. Pierre and Paul Mallet followed the entire length of the river in 1740, as did another expedition led by Fabry de la Bruyère in 1741. The Louisiana Purchase in 1803 resulted in all of the land east of the New Mexico border being acquired by the United States.

In 1818, the Quapaw tribe ceded all its land north of the Canadian to the United States, thus making this river the effective southern boundary of the new nation. In 1825, the Osage ceded their claims to land along the river. The Canadian was designated as the boundary between the Creek-Seminole lands on the north side and the Choctaw (and later the Chickasaw) on the south side. Major Stephen H. Long led an expedition up the Canadian River in 1821. He proclaimed the land along the river as the Great American Desert. Despite this assessment, trading posts were established along the river, starting with Edwards' Post at the mouth of Little River. Camp Holmes was established by Colonel Henry Dodge's Dragoons in 1834. Captain Nathan Boone led a dragoon troop up the river to the 100th meridian, which was then the western border of the United States. See also: Stephen H. Long's Expedition of 1820