Indian Territory, a former unorganized territory of the United States set aside for the Indian tribes being removed from east of the Mississippi River. An act of Congress of 1830 provided that it should include all areas not a part of any state or organized territory. An act of 1854 organizing the territories of Kansas and Nebraska reduced Indian Territory to the limits of the present state of Oklahoma without the Panhandle.

The territory was used for Indian resettlement from 1820, when lands were assigned to the Choctaws. During the next two decades the Cherokees, Chickasaws, Creeks, and Seminoles were also moved to it. They set up their own governments, or nations, and became known as the Five Civilized Tribes. Later they gave up their western territory to other displaced tribes.

An unassigned tract in the center of the territory was opened to white settlement in 1889 and organized as part of Oklahoma Territory in 1890. Additional tracts were purchased from the Indians; by 1901 half of the former Indian Territory was part of Oklahoma Territory. In 1907 the two territories were united as the state of Oklahoma.

Oil discoveries in the 20th century brought wealth to many Oklahoma Indians with holdings in tribal lands.