The Chagossians, also known as Chagos Islanders and Îlois (French: [i.lwa]), are an Afro-Asian ethnic group originating from freed African slaves as well as people of Asian (South Asian and Malay) descent brought to the Chagos Islands, specifically Diego Garcia, Peros Banhos, and the Salomon island chain, in the late 18th century. The Chagossians are a mix of African, South Asian and Malay descent. The French brought some to the Chagos Islands as slaves from Mauritius in 1776. Others arrived as fishermen, farmers, and coconut plantation workers during the 19th century. Under international law, they are the indigenous peoples of the Chagos archipelago, as they are descended from the earliest human settlers of the islands. Most Chagossians now live in Mauritius, Seychelles, and the United Kingdom after the forced removal by the British government in the late 1960s and early 1970s so that Diego Garcia, the island where most Chagossians lived, could serve as the location for a joint United Kingdom–United States military base. Since 1971, no Chagossians have been allowed to live on the island of Diego Garcia, nor anywhere in the Chagos Archipelago, despite many of the once-inhabited islands being over 160 kilometres (86 nmi; 99 mi) away from Diego Garcia.

In 2026, four Chagossians linked to the self-declared Chagossian Government returned to Île du Coin to reestablish the settlement there, without seeking government permission.

The Chagossians speak Chagossian Creole, a French-based creole language whose vocabulary also incorporates words originating in various African and Asian languages and is part of the Bourbonnais Creole family. Chagossian Creole is still spoken by some of their descendants in Mauritius and the Seychelles. Chagossian people living in the UK speak English. Some settled in the town of Crawley in West Sussex, and the Chagossian community there numbered approximately 3,000 in 2016, which increased to 3,500 in 2024. Manchester also has a Chagossian community, which includes artist Audrey Albert.

In 2016, the British government rejected the right of the Chagossians to return to the islands after a 45-year legal dispute. In 2019, the International Court of Justice issued an advisory opinion stating that the United Kingdom did not have sovereignty over the Chagos Islands and that the administration of the archipelago should be handed over "as rapidly as possible" to Mauritius. Since this, the United Nations General Assembly and the International Tribunal for the Law of the Sea have reached similar decisions. China abstained in the 2019 UN vote, which was a step towards reaching an agreement to return the Chagos Archipelago to Mauritius.

In October 2024, the UK agreed to hand over the Chagos Islands to Mauritius and stated that Mauritius "will now be free to implement a programme of resettlement on the islands of the Chagos Archipelago, other than Diego Garcia". The UK will also set up a trust fund for the scattered Chagossian diaspora, now numbering 10,000. In 2021, Mauritius amended its Criminal Code to outlaw "Misrepresenting the sovereignty of Mauritius over any part of its territory", with the penalty of a fine or jail term up to 10 years but only for a "person who [is] acting under the authority or instructions of, or pursuant to a contract with, or with the direct or indirect financial support of, a foreign State or any organ or agency of such a State".

In 1793, when the first successful colony was founded on Diego Garcia, coconut plantations were established on many of the atolls and isolated islands of the archipelago. Initially the workers were enslaved Africans,^{[clarification needed]} but after 1840 they were freemen, many of whom were descended from those earlier enslaved. They formed an inter-island culture called Ilois (a French Creole word meaning Islanders).

In 1965, as part of a deal to grant Mauritian independence, the UK separated the Chagos Archipelago, at the time a part of its Mauritius territory, from the colony and reorganized it as the British Indian Ocean Territory. The UK also labelled the Chagossians, whose ancestral links to the territory go back to the late 18th century, as “transient workers” to avoid breaching International Law. The territory's new constitution was set out in a statutory instrument imposed unilaterally with no referendum or consultation with the Chagossians and it envisaged no democratic institutions. On 16 April 1971, the United Kingdom issued a policy called BIOT Immigration Ordinance #1 which made it a criminal offence for those without military clearance to be on the islands without a permit.

Between 1967 and 1973, the Chagossians, then numbering over 1,000 people, were expelled by the British government, first to the island of Peros Banhos, 100 miles (160 km) away from their homeland, and then, in 1973, to Mauritius. A number of Chagossians who were evicted reported that they were threatened with being shot or bombed if they did not leave the island. One old man reported to The Washington Post journalist David Ottaway that an American official told him, "If you don't leave you won't be fed any longer." BIOT commissioner Bruce Greatbatch later ordered all dogs on the island to be killed. Meanwhile, food stores on the island were allowed to deplete in order to pressure the remaining inhabitants to leave. The Chagossians owned no real property on the islands and lived in housing provided for farm workers by the absentee landowners of the plantations. The forced expulsion of the Chagossians after the acquisition of the plantations from their absentee landlords by the British Government was for the purpose of establishing a United States air and naval base on Diego Garcia, with a population of between 3,000 and 5,000 U.S. soldiers and support staff, as well as a few troops from the United Kingdom. Their exile is referred to as the "dérasiné" in the Chagossian language.