The Umatilla Indian Reservation is an Indian reservation in the Pacific Northwest of the United States. It was created by The Treaty of 9 June 1855 between the United States and members of the Walla, Cayuse, and Umatilla tribes. It lies in northeastern Oregon, east of Pendleton. The reservation is mostly in Umatilla County, with a very small part extending south into Union County. It is managed by the three Confederated Tribes of the Umatilla Indian Reservation.

An American Indian reservation is an area of land held and governed by a U.S. federal government-recognized Native American tribal nation, whose government is autonomous, subject to regulations passed by the United States Congress and administered by the United States Bureau of Indian Affairs, and not to the U.S. state government in which it is located. Some of the country's 574 federally recognized tribes govern more than one of the 326 Indian reservations in the United States, while some share reservations, and others have no reservation at all. Historical piecemeal land allocations under the Dawes Act facilitated sales to non–Native Americans, resulting in some reservations becoming severely fragmented, with pieces of tribal and privately held land being treated as separate enclaves. This jumble of private and public real estate creates significant administrative, political, and legal difficulties.