The Platte River is an approximately 310 mi. (499 km) long river in the Western United States. It is a tributary to the Missouri River, which in turn is a tributary to the Mississippi River. Platte River being one of the most significant river systems in the watershed of the Missouri, it drains a large portion of the central Great Plains in Nebraska and the eastern Rocky Mountains in Colorado and Wyoming. The river was highly significant in the westward expansion of the United States, providing the route for several major westward trails, including the Oregon Trail and the Mormon Trail. In the 18th century, it was also known among French fur trappers who explored it as the Nebraska River.

The Platte River is formed in western Nebraska east of the city of North Platte by the confluence of its two affluents, the South Platte and the North Platte rivers, both of which rise in the eastern Rockies near the Continental Divide. It flows in a large arc, southeast then northeast, across Nebraska south of the Sandhills region, passing Gothenburg, Cozad, Kearney, and Grand Island. It is joined by the Loup River 5 miles (8.0 km) southeast of Columbus and flows east past North Bend then to Fremont, then south, passing south of Omaha and joining the Missouri 5 miles (8.0 km) north of Plattsmouth. Combined with the length of the North Platte, the Platte stretches over 900 miles (1,448 km), with a drainage basin of 90,000 square miles (233,099 km²).

The Platte drains one of the most arid areas of the Great Plains and thus its flow is considerably lower than rivers of comparable length in North America. For much of its length, it is a classic wide and shallow braided stream. During pioneer days, the common humorous description was that the Platte was "a mile wide at the mouth, but only six inches deep." 49ers said it was "too thick to drink, too thin to plow". In western Nebraska, the banks and riverbed of the Platte provide a green oasis amid an otherwise semi-arid region of North America. The central Platte River valley is an important stopover for migratory water birds, such as the Whooping Crane and Sandhill Crane, in their yearly traversal of the Central Flyway.

This river has shrunk significantly in the past 70 years. This reduction in size is attributed in part to irrigation, and to a much greater extent to the waters diverted and used by the growing population of Colorado, which has outstripped the ability of its groundwater to sustain them.

The first European to discover the Platte was the French explorer Étienne de Veniard, sieur de Bourgmont in 1714, who named it the Nebraskier, an Oto word meaning "flat water". The French word for flat, platte, was later applied. The river provided valuable transportation for the French trade in furs with the Pawnee and Oto indians

The Platte lay in a gray area between Spanish and French claims in the Great Plains. Joseph Naranjo, a black explorer, had also encountered the Platte, and later guided the Villasur expedition there to stop French expansion. Theirs was the deepest penetration of Spanish exploration into the central plains.