An Indian reservation is an area of land held and governed by a U.S. federal government-recognized Native American tribal nation whose government is semi-sovereign subject to regulations passed by the United States Congress 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.

The total area of all reservations is ,56,200,000 acres (22,700,000 ha; 87,800 sq mi; 227,000 km²) approximately 2.3% of the total area of the United States and about the size of the state of Idaho. While most reservations are small compared to the average U.S. state, twelve Indian reservations are larger than the state of Rhode Island. The largest reservation, the Navajo Nation Reservation, is similar in size to the state of West Virginia. Reservations are unevenly distributed throughout the country, the majority being situated west of the Mississippi River and occupying lands that were first reserved by treaty (Indian Land Grants) from the public domain.

Because recognized Native American nations possess tribal sovereignty, albeit of a limited degree, laws within tribal lands may vary from those of the surrounding and adjacent states. For example, these laws can permit casinos on reservations located within states which do not allow gambling, thus attracting tourism. The tribal council generally has jurisdiction over the reservation, not the U.S. state it is located in or the U.S. federal government. Different reservations have different systems of government, which may or may not replicate the forms of government found outside the reservation. Most Native American reservations were established by the federal government but a small number, mainly in the East, owe their origin to state recognition.

The term "reservation" is a legal designation. It comes from the conception of the Native American nations as independent sovereigns at the time the U.S. Constitution was ratified. Thus, early peace treaties (often signed under conditions of duress or fraud), in which Native American nations surrendered large portions of their land to the United States, designated parcels which the nations, as sovereigns, "reserved" to themselves, and those parcels came to be called "reservations". The term remained in use after the federal government began to forcibly relocate nations to parcels of land to which they often had no historical or cultural connection. Compared to other population centers in the U.S., reservations are disproportionately located on or near toxic sites hazardous to the health of those living or working in close proximity, including nuclear testing grounds and contaminated mines.

Today the majority of American Indians and Alaska Natives live outside the reservations, mainly in the larger western cities such as Phoenix and Los Angeles. In 2012, there were over 2.5 million Native Americans, with 1 million living on reservations.