The Osage Nation ( OH-sayj) (Osage: 𐓁𐒻 𐓂𐒼𐒰𐓇𐒼𐒰͘ (Ni Okašką), "People of the Middle Waters") is a Midwestern Native American tribe of the Great Plains. The tribe developed in the Ohio and Mississippi river valleys around 700 BC along with other groups of its language family. They migrated west after the 17th century, settling near the confluence of the Missouri and Mississippi rivers, as a result of Iroquois invading the Ohio Valley in a search for new hunting grounds.

The term "Osage" is a French version of the tribe's name, which can be roughly translated as "calm water". The Osage people refer to themselves in their indigenous Dhegihan Siouan language as 𐓏𐒰𐓓𐒰𐓓𐒷 (Wazhazhe), or "Mid-waters".

By the early 19th century, the Osage had become the dominant power in the region, feared by neighboring tribes. The tribe controlled the area between the Missouri and Red rivers, the Ozarks to the east and the foothills of the Wichita Mountains to the south. They depended on nomadic buffalo hunting and agriculture.

The 19th-century painter George Catlin described the Osage as "the tallest race of men in North America, either red or white skins; there being ... many of them six and a half, and others seven feet." The missionary Isaac McCoy described the Osage as an "uncommonly fierce, courageous, warlike nation" and said they were the "finest looking Indians I have ever seen in the West".

In the Ohio Valley, the Osage originally lived among speakers of the same Dhegihan language stock, such as the Kansa, Ponca, Omaha, and Quapaw. Researchers believe that the tribes likely became differentiated in languages and cultures after leaving the lower Ohio country. The Omaha and Ponca settled in what is now Nebraska; the Kansa in Kansas; and the Quapaw in Arkansas.

In the 19th century, the Osage were forced by the United States to remove from Kansas to Indian Territory (present-day Oklahoma), and the majority of their descendants live in Oklahoma. In the early 20th century, oil was discovered on their land. They had retained communal mineral rights during the allotment process, and many Osage became wealthy through returns from leasing fees generated by their headrights. However, during the 1920s and what was known as the Reign of Terror, they suffered manipulation, fraud, and numerous murders by outsiders eager to take over their wealth.

In 2011, the nation gained a settlement from the federal government after an 11-year legal struggle over long mismanagement of their oil funds. In the 21st century, the federally recognized Osage Nation has approximately 20,000 enrolled members, 6,780 of whom reside in the tribe's jurisdictional area. Members also live outside the nation's tribal land in Oklahoma and in other states around the country. The tribe is bordered by the Cherokee Nation to the east, the Muscogee Nation and the Pawnee Nation to the south, and the Kaw Nation and Oklahoma proper to the west.