The Missouri River is the longest river in North America and a major waterway of the central United States. Rising in the Rocky Mountains of western Montana, the Missouri flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river takes drainage from a sparsely populated, semi-arid watershed of more than half a million square miles (1,300,000 km²), which includes parts of ten U.S. states and two Canadian provinces.

For over 12,000 years, people have depended on the Missouri and its tributaries as a source of sustenance and transportation. More than ten major groups of Native Americans populated the watershed, most leading a nomadic lifestyle and dependent on enormous buffalo herds that once 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 finally becoming part of the United States through the Louisiana Purchase. The Missouri was long believed to be part of the Northwest Passage – a water route from the Atlantic to the Pacific – but when Lewis and Clark became the first to travel the river's entire length, they confirmed the mythical pathway to be no more than a legend.

Missouri (ⁱ/mɨˈzʊəri/) (nickname The Show Me State) is a U.S. state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. Missouri is the 21st most extensive and the 18th most populous of the 50 United States. Missouri comprises 114 counties and the independent city of St. Louis. The four largest urban areas are St. Louis, Kansas City, Springfield, and Columbia. Missouri's capital is Jefferson City. The land that is now Missouri was acquired from France as part of the Louisiana Purchase and became known as the Missouri Territory. Part of the Territory was admitted into the union as the 24th state on August 10, 1821.

Missouri generally mirrors the demographic, economic and political makeup of America and has long been considered a political bellwether state. With the exceptions of the Presidential elections of 1956 and 2008, Missouri's results have accurately predicted the next President of the United States in every election since 1904. It has both Midwestern and Southern cultural influences, reflecting its history as a border state. It is also a transition between the Eastern and Western United States, as St. Louis is often called the "western-most Eastern city" and Kansas City the "eastern-most Western city". Missouri's geography is highly varied. The northern part of the state lies in dissected till plains while the southern part lies in the Ozark Mountains (a dissected plateau), with the Missouri River dividing the two. The state lies at the intersection of the three greatest rivers of North America, with the confluence of the Mississippi and Missouri Rivers near St. Louis, and the confluence of the Ohio River with the Mississippi north of the Bootheel. The starting points of the Pony Express Trail and Oregon Trail were both in Missouri. The mean center of United States population as of the 2010 Census is at the town of Plato in Texas County, Missouri.