Diego García is a coral atoll and the largest island in terms of land area, of the Chagos Archipelago. It is part of the British Indian Ocean Territory. The island is located in the Indian Ocean, about 1,600 km (1,000 mi) south of the southern coast of India. The closest other countries to Diego Garcia are Sri Lanka and Maldives.

The Portuguese were the first Europeans to reach and explore the island of Diego Garcia, with the discovery attributed to the navigator Pêro de Mascarenhas while sailing in a fleet under Dom Garcia de Noronha's leadership during 1512 and 1513.

In the 1960s, the Chagos archipelago was secretly leased to the United Kingdom and detached from Mauritius with the intention of expelling its entire population and establishing a military base. In 1971 the United Kingdom and United States entered an agreement under which the latter would set up a military base in Diego Garcia.

Since then, the United Kingdom enforced the highly controversial depopulation of Diego Garcia, forcing the deportation of the island's entire 2,000 inhabitants, descendants of African slaves and Hindu laborers brought to the islands by the French in the 18th century, to the surrounding islands including Mauritius, located 1,200 miles away. In their place a joint British-American military base was established.

This island has one of the five monitoring stations assisting the operation of the Global Positioning System, the others being on Ascension Island, Hawaii, Kwajalein Atoll and in Colorado Springs.

It is covered in luxuriant tropical vegetation. It is 60 kilometres (37 mi) long, with a maximum elevation of 6.7 metres (22 ft), and nearly encloses a lagoon about 19 kilometres (12 mi) long and up to 8 kilometres (5.0 mi) wide. Depths in the lagoon extend to 30 metres (98 ft), and numerous coral heads present hazards to navigation. Shallow reefs surround the island on the ocean side. The channel and anchorage area are dredged, while the old turning basin can also be used.