The Missouria or Missouri (in their own language, Nutachi, also spelled Niutachi) are a Native American tribe that originated in the Great Lakes region of what is now the United States before European contact. The tribe belongs to the Chiwere division of the Siouan language family, together with the Ho-Chunk, Iowa, and Otoe.

Throughout the 17th and 18th centuries, the tribe lived in bands near the mouth of the Grand River and Missouri rivers at its confluence with the Missouri River, the mouth of the Missouri at its confluence with the Mississippi River, and in present-day Saline County, Missouri. Since Indian removal, they live primarily in Oklahoma. They are federally recognized as the Otoe-Missouria Tribe of Indians, headquartered in Red Rock, Oklahoma.

French colonists adapted a form of the Illinois language-name for the people: Wimihsoorita, which translates as "One who has dugout canoes". In their own Siouan language, the Missouri call themselves Nutachi, also spelled Niutachi, meaning "People of the River Mouth." The Osage called them the Waçux¢a, and the Quapaw called them the Wa-ju'-xd¢ǎ.

The state of Missouri and the Missouri River are named for the tribe.

The tribe's oral history tells that they once lived north of the Great Lakes, where they were part of a larger tribe that included the Ho-Chunk, Iowa, and Otoe. They began migrating south in the 16th century.

The beginning of the 17th century, the Missouria lived near the confluence of the Grand and Missouri rivers, where they settled through the 18th century. Later, their oral history says that they split from the Otoe tribe, which belongs to the same Chiwere branch of the Siouan language, because of a love affair between the children of two tribal chiefs.

The 17th century brought hardships to the Missouria. The Sauk and Meskwaki frequently attacked them. Their society was even more disrupted by the high fatalities from epidemics of smallpox and other Eurasian infectious diseases that accompanied contact with Europeans. The French explorer Jacques Marquette contacted the tribe in 1673 and paved the way for trade with the French.

The Missouria migrated west of the Missouri River into Osage territory. During this time, they acquired horses and hunted bison. The French explorer Étienne de Veniard, Sieur de Bourgmont visited the people in the early 1720s. He married the daughter of a Missouria chief. They settled nearby, and Veniard created alliances with the people. He built Fort Orleans in 1723 as a trading post near present-day Brunswick, Missouri. It was occupied until 1726.