Cornish people or the Cornish (Cornish: Kernowyon, Old English: Cornƿīelisċ) are an ethnic group native to, or associated with Cornwall and a recognised national minority in the United Kingdom, which (like the Welsh and Bretons) can trace its roots to the ancient Britons who inhabited Great Britain from somewhere between the 11th and 7th centuries BC and inhabited Britain at the time of the Roman conquest. Many in Cornwall today continue to assert a distinct identity separate from or in addition to English or British identities. Cornish identity has also been adopted by some migrants into Cornwall, as well as by emigrant and descendant communities from Cornwall, the latter sometimes referred to as the Cornish diaspora. Although not included as a tick-box option in the UK census, the numbers of those writing in a Cornish ethnic and national identity are officially recognised and recorded.

Throughout classical antiquity, the ancient Celtic Britons formed a series of tribes, kingdoms, cultures and identities throughout Great Britain; the Dumnonii and Cornovii were the Celtic tribes who inhabited what was to become Cornwall during the Iron Age, Roman and post-Roman periods. The name Cornwall and its demonym Cornish are derived from the Celtic Cornovii tribe. The Anglo-Saxon invasion and settlement of Britain starting from the late 5th and early 6th centuries and the arrival of Scots from Ireland during the same period gradually restricted the Romano-British culture and Brittonic language into parts of the north and west of Great Britain by the 10th century, whilst the inhabitants of southern, central and eastern Britain became English and much of the north became Scottish. The Cornish people, who shared the Brythonic language with the Welsh, Cumbrics and Picts, and also the Bretons who had migrated across the sea to escape the Anglo-Saxon invasions, were referred to in the Old English language as the "Westwalas" meaning West Welsh. The Battle of Deorham between the Britons and Anglo-Saxons is thought to have resulted in a loss of land links with the people of Wales.

The Cornish people and their Brythonic Cornish language experienced a slow process of anglicisation and attrition during the medieval and early modern periods. By the 18th century, and following the creation of the Kingdom of Great Britain, the Cornish language and to some degree identity had faded, largely replaced by the English language (albeit Cornish-influenced West Country dialects and Anglo-Cornish) or British identity. A Celtic revival during the early-20th century enabled a cultural self-consciousness in Cornwall that revitalised the Cornish language and roused the Cornish to express a distinctly Brittonic Celtic heritage. The Cornish language was granted official recognition under the European Charter for Regional or Minority Languages in 2002, and in 2014 the Cornish people were recognised and afforded protection by the UK Government under the Framework Convention for the Protection of National Minorities.

In the 2021 census, the population of Cornwall, including the Isles of Scilly, was recorded as 570,300. The Cornish self-government movement has called for greater recognition of Cornish culture, politics, and language, and urged that Cornish people be accorded greater status, exemplified by the call for them to be one of the listed ethnic groups in the United Kingdom Census 2011 form.

Both geographic and historical factors distinguish the Cornish as an ethnic group further supported by identifiable genetic variance between the populations of Cornwall, neighbouring Devon and England as published in a 2012 Oxford University study. Throughout medieval and Early Modern Britain, the Cornish were at some points accorded the same status as the English and Welsh and considered a separate race or nation, distinct from their neighbours, with their own language, society and customs. A process of Anglicisation between 1485 and 1700 led to the Cornish adopting English language, culture and civic identity, a view reinforced by Cornish historian A. L. Rowse who said they were gradually "absorbed into the mainstream of English life". Although "decidedly modern" and "largely retrospective" in its identity politics, Cornish and Celtic associations have advanced the notion of a distinct Cornish national and ethnic identity since the late 20th century. In the United Kingdom Census 2001, despite no explicit "Cornish" option being available, approximately 34,000 people in Cornwall and 3,500 people elsewhere in the UK—a combined total equal to nearly 7 per cent of the population of Cornwall—identified themselves as ethnic Cornish by writing this in under the "other" ethnicity option. The census figures show a change in identity from West to East, in Penwith 9.2 per cent identified as ethnically Cornish, in Kerrier it was 7.5 per cent, in Carrick 6.6 per cent, Restormel 6.3 per cent, North Cornwall 6 per cent, and Caradon 5.6 per cent. Weighting of the 2001 Census data gives a figure of 154,791 people with Cornish ethnicity living in Cornwall.

The Cornish have been described as "a special case" in England, with an "ethnic rather than regional identity". Structural changes to the politics of the United Kingdom, particularly the European Union and devolution, have been cited as the main stimulus to "a growing interest in Cornish identity and distinctiveness" in late-20th century Britain. The British are the citizens of the United Kingdom, a people who by convention consist of four national groups: the English, Northern Irish, Scots and Welsh. In the 1990s it was said that the notion that the Cornish are to be classified as a nation comparable to the English, Irish, Scots and Welsh, "has practically vanished from the popular consciousness" outside Cornwall, and that, despite a "real and substantive" identity, the Cornish "struggle for recognition as a national group distinct from the English". However, in 2014, after a 15-year campaign, the UK government officially recognised the Cornish as a national minority under the Council of Europe's Framework Convention for the Protection of National Minorities, giving the Cornish the same status as the Welsh, Scots and Irish within the UK.

Inhabitants of Cornwall may have multiple political allegiances, adopting mixed, dual or hyphenated identities such as "Cornish first and British second", "Cornish and British and European", or, like Phil Vickery (a rugby union prop for the England national rugby union team and British and Irish Lions), describe themselves as "Cornish" and "English". Meanwhile, another international rugby union player, Josh Matavesi, describes himself as Cornish-Fijian and Cornish not English.

A survey by Plymouth University in 2000 found that 30% of children in Cornwall felt "Cornish, not English". A 2004 survey on national identity by the finance firm Morgan Stanley found that 44% of respondents in Cornwall saw themselves as Cornish rather than British or English. A 2008 University of Exeter study conducted in 16 towns across Cornwall found that 59% felt themselves to be Cornish and 41% felt "More Cornish than English", while for over a third of respondents the Cornish identity formed their primary national identity. Genealogy and family history were considered to be the chief criteria for 'being' Cornish, particularly among those who possessed such ties, while being born in Cornwall was also held to be important.