The Missouri River is a river in the Central and Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Mountains of southwestern Montana, then flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river drains semi-arid watershed of more than 500,000 square miles (1,300,000 km²), which includes parts of ten U.S. states and two Canadian provinces. Although a tributary of the Mississippi, the Missouri River is slightly longer and carries a comparable volume of water, though a fellow tributary (Ohio River) carries more water. When combined with the lower Mississippi River, it forms the world's fourth-longest river system.

For over 12,000 years, people have depended on the Missouri River and its tributaries as a source of sustenance and transportation. More than ten major groups of Native Americans populated the watershed, with most leading a nomadic lifestyle and dependent on enormous bison herds that roamed through the Great Plains. The first Europeans encountered the river in the late seventeenth century, and the region passed through Spanish and French hands before becoming part of the United States through the Louisiana Purchase.

The Missouri River was one of the main routes for the westward expansion of the United States during the 19th century. The growth of the fur trade in the early 19th century laid much of the groundwork as trappers explored the region and blazed trails. Pioneers headed west en masse beginning in the 1830s, first by covered wagon, then by the growing numbers of steamboats that entered service on the river. Conflict between settlers and Native Americans in the watershed led to some of the most longstanding violence of the American Indian Wars.

During the 20th century, the Missouri River basin was extensively developed for irrigation, flood control, and the generation of hydroelectric power. Fifteen dams impound the main stem of the river, with hundreds more on tributaries. The Missouri River's reservoirs include the largest, second-largest, and fourth-largest artificial lakes in the United States by surface area: Lake Sakakawea, Lake Oahe, and Fort Peck Lake. Meanders have been cut off and the river channelized to improve navigation, reducing its length by almost 200 miles (320 km) from pre-development times. Although the lower Missouri valley is now a populous and highly productive agricultural and industrial region, heavy development has taken its toll on wildlife and fish populations as well as water quality.

From the Rocky Mountains, three streams rise to form the headwaters of the Missouri River:

The Missouri River officially starts at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks, Montana, and is joined by the Gallatin a mile (1.6 km) downstream. It then passes through Canyon Ferry Lake, a reservoir west of the Big Belt Mountains. Issuing from the mountains near Cascade, the river flows northeast to the city of Great Falls, where it drops over the Great Falls of the Missouri, a series of five substantial waterfalls. It then winds east through a scenic region of canyons and badlands known as the Missouri Breaks, receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River. Farther on, the river passes through the Fort Peck Dam, and immediately downstream, the Milk River joins from the north.

Flowing eastward through the plains of eastern Montana, the Missouri receives the Poplar River from the north before crossing into North Dakota where the Yellowstone River, its greatest tributary by volume, joins from the southwest. At the confluence, the Yellowstone is actually the larger river. The Missouri then meanders east past Williston and into Lake Sakakawea, the reservoir formed by Garrison Dam. Below the dam the Missouri receives the Knife River from the west and flows south to Bismarck, the capital of North Dakota, where the Heart River joins from the west. It slows into the Lake Oahe reservoir just before the Cannonball River confluence. While it continues south, eventually reaching Oahe Dam in South Dakota, the Grand, Moreau and Cheyenne Rivers all join the Missouri from the west.

The Missouri makes a bend to the southeast as it winds through the Great Plains, receiving the Niobrara River and many smaller tributaries from the southwest. It then proceeds to form the boundary of South Dakota and Nebraska and is joined by the James River from the north. At Sioux City the Big Sioux River comes in from the north, after which the Missouri forms the Iowa–Nebraska boundary. It flows south to the city of Omaha where it receives its longest tributary, the Platte River, from the west. Downstream, it begins to define the border between the states of Nebraska and Missouri, then flows between the states of Missouri and Kansas. The Missouri swings east at Kansas City, where the Kansas River enters from the west, and so on into north-central Missouri. To the east of Kansas City, the Missouri receives, on the left side, the Grand River. It passes south of Columbia and receives the Osage and Gasconade Rivers from the south downstream of Jefferson City. The river then rounds the northern side of St. Louis to join the Mississippi River on the border between Missouri and Illinois.