Indian Territory and the Indian Territories are terms that generally described an evolving land area set aside by the United States government for the relocation of Native Americans who held original Indian title to their land as an independent nation. The concept of an Indian territory was an outcome of the U.S. federal government's 18th- and 19th-century policy of Indian removal. After the American Civil War (1861–1865), the policy of the U.S. government was one of assimilation.

Indian Territory later came to refer to an unorganized territory whose general borders were initially set by the Nonintercourse Act of 1834, and was the successor to the remainder of the Missouri Territory after Missouri received statehood. The borders of Indian Territory were reduced in size as various Organic Acts were passed by Congress to create organized territories of the United States. The 1906 Oklahoma Enabling Act created the single state of Oklahoma by combining Oklahoma Territory and Indian Territory, ending the existence of an unorganized independent Indian Territory as such, and formally incorporating the tribes and residents into the United States.

Before Oklahoma statehood, Indian Territory from 1890 onward comprised the territorial holdings of the Cherokee, Choctaw, Chickasaw, Creek, Seminole, and other displaced Eastern American tribes. Indian reservations remain within the boundaries of U.S. states, but are largely exempt from state jurisdiction. The term "Indian country" today is used to signify lands under the control of Native nations, including Indian reservations, trust lands on Oklahoma Tribal Statistical Area, or, more casually, to describe anywhere large numbers of Native Americans live.

Indian Territory, also known as the Indian Territories and the Indian Country, was land in the United States reserved for the forced resettlement of Native Americans. As such, it was not a traditional territory for the tribes settled upon it. The general borders were set by the Indian Intercourse Act of 1834. The territory was located in the Central United States.

While Congress passed several Organic Acts that provided a path for statehood for much of the original Indian Country, Congress never passed an Organic Act for the Indian Territory. Indian Territory was never an organized territory of the United States. In general, tribes could not sell land to non-Indians (Johnson v. McIntosh). Treaties with the tribes restricted entry of non-Indians into tribal areas; Indian tribes were largely self-governing, were suzerain nations, with established tribal governments and well established cultures. The region never had a formal government until after the American Civil War.

After the Civil War, the Southern Treaty Commission re-wrote treaties with tribes that sided with the Confederacy, reducing the territory of the Five Civilized Tribes and providing land to resettle Plains Indians and tribes of the Midwestern United States. These re-written treaties included provisions for a territorial legislature with proportional representation from various tribes.

In time, the Indian Territory was reduced to what is now Oklahoma. The Organic Act of 1890 reduced Indian Territory to the lands occupied by the Five Civilized Tribes and the Tribes of the Quapaw Indian Agency (at the borders of Kansas and Missouri). The remaining western portion of the former Indian Territory became the Oklahoma Territory.

The Oklahoma Organic Act applied the laws of Nebraska to the organized Oklahoma Territory, and the laws of Arkansas to the still unorganized Indian Territory, since for years the federal U.S. District Court on the eastern borderline in Ft. Smith, Arkansas had criminal and civil jurisdiction over the territory.