The British diaspora consists of people of English, Scottish, Welsh, Northern Irish, Cornish, Manx and Channel Islands ancestral descent who live outside of the United Kingdom and its Crown Dependencies.

In 2008, the United Kingdom's Foreign and Commonwealth Office estimated that at least 80% of New Zealanders had some British ancestry, however at the 2018 census only 70% of New Zealanders identified as having some European ancestry. Up to 76% of Australians, 48% of Canadians, 33% of Americans, and 3% of South Africans have ancestry from the British Isles. Additionally, at least 270,000 Argentines have some British ancestry. More than 300,000 Anglo-Indians have some British ancestry, but comprise less than 0.1% of India's population.

The British diaspora includes about 200 million people worldwide. Other countries with over 100,000 British expatriates include the Republic of Ireland, Spain, France, Germany, and the United Arab Emirates.

The first documented exodus of Britons began during the Anglo-Saxon invasion of Great Britain. A large number of Brythonic-speaking Celts fled or migrated to what is now Brittany on the coast of France, becoming the Bretons.

The second large-scale British migration came following the Norman Conquest of England, leading to a displacement of English people, mostly dispossessed nobility. They settled in neighboring regions including Ireland and Scandinavia, and as far east as Crimea and Anatolia in the Byzantine Empire. Englishmen eventually replaced Scandinavians as the main source of recruitment for the Byzantine Emperor's personal Varangian Guard.

After the Age of Discovery, the various peoples of the British Isles, and especially the English, were among the earliest and by far the largest communities to emigrate out of Europe. Indeed, the British Empire's expansion during the first half of the 19th century saw an extraordinary dispersion of the British people, with particular concentrations in Australasia and North America.

The British Empire was "built on waves of migration overseas by British people", who left Great Britain, later the United Kingdom, and reached across the globe and permanently affected population structures in three continents. As a result of the British colonisation of the Americas, what became the United States was "easily the greatest single destination of emigrant British", but in what would become the Commonwealth of Australia the British experienced a birth rate higher than anything seen before, which together with continuing British immigration resulted in a huge outnumbering of indigenous Australians.

In colonies such as Southern Rhodesia, British Hong Kong, Singapore, Jamaica, Barbados, Malaysia, and the Cape Colony, permanently resident British communities were established, and while never more than a numerical minority, these Britons exercised a dominant influence upon the culture and politics of those lands. In Australia, Canada, and New Zealand, people of British origin came to constitute the majority of the population, contributing to these states becoming integral to the Anglosphere.