Diego Garcia is an island of the British Indian Ocean Territory (BIOT), a disputed overseas territory of the United Kingdom also claimed by Mauritius. It is a militarised atoll just south of the equator in the central Indian Ocean, and the largest of the 60 small islands of the Chagos Archipelago. Portuguese sailors under Pedro Mascarenhas were the first Europeans to discover the island, finding it uninhabited in 1512. After a 1786 British colony failed, the French began using the island as a leper colony and, starting in 1793, coconut cultivation by slave labour. It was transferred to British rule after the Napoleonic Wars. It was one of the "Dependencies" of the British Colony of Mauritius until the Chagos Islands were detached for inclusion in the newly created British Indian Ocean Territory in 1965.

In 1966, the population of the island was 924.: par 23  These people were employed as contract farm workers primarily on copra plantations owned by the Chagos-Agalega company. Although local plantation managers commonly allowed pensioners and the disabled to remain in the islands and continue to receive housing and rations in exchange for light work, children after the age of 12 were required to work. In 1964, only 3 of a population of 963 were unemployed. In April 1967, the government of the British Indian Ocean Territory bought out Chagos-Agalega for £600,000, thus becoming the sole property owner in the BIOT. The Crown immediately leased back the properties to Chagos-Agalega but the company terminated the lease at the end of 1967.

Between 1968 and 1973, the Chagossian (Îlois) inhabitants were forcibly expelled from Diego Garcia by the UK government so that a joint US/UK military base could be established on the island. Many were deported to Mauritius and the Seychelles, following which the United States built the large Naval Support Facility Diego Garcia, which has been in continuous operation since then. In 2019, this action and continued British administration of the archipelago were deemed illegal by the International Court of Justice in The Hague, a ruling the United Nations General Assembly supported. However, the British dismissed this ruling as not legally binding. In 2023, Human Rights Watch condemned the action as a crime against humanity. As of August 2018, Diego Garcia is the only inhabited island of the BIOT; the population is composed of military personnel and supporting contractors. It is one of two critical US bomber bases in the Indo-Pacific region, along with Andersen Air Force Base in Guam.

The atoll is located 3,535 km (2,197 mi) east of Tanzania's coast, 1,796 km (1,116 mi) south-southwest of the southern tip of India (at Kanyakumari), and 4,723 km (2,935 mi) west-northwest of the west coast of Australia (at Cape Range National Park, Western Australia). Diego Garcia lies at the southernmost tip of the Chagos-Laccadive Ridge, a vast underwater mountain range with peaks consisting of coral reefs, atolls, and islands comprising Lakshadweep, the Maldives, and the Chagos Archipelago. Local time is UTC+6 year-round.