The Cornish dialect (also known as Cornish English, Anglo-Cornish or Cornu-English) is a dialect of English spoken in Cornwall by Cornish people. Dialectal English spoken in Cornwall is to some extent influenced by Cornish grammar, and often includes words derived from the Cornish language. The Cornish language is a Celtic language of the Brythonic branch, as are the Welsh and Breton languages. In addition to the distinctive words and grammar, there are a variety of accents found within Cornwall from the north coast to that of the south coast and from east to west Cornwall. Typically, the accent is more divergent from Standard British English the further west through Cornwall one travels. The speech of the various parishes being to some extent different from the others was described by John T. Tregellas and Thomas Quiller Couch towards the end of the 19th century. Tregellas wrote of the differences as he understood them and Couch suggested the parliamentary constituency boundary between the East and West constituencies, from Crantock to Veryan, as roughly the border between eastern and western dialects. To this day, the towns of Bodmin and Lostwithiel as well as Bodmin Moor are considered the boundary.

The first speakers of English resident in Cornwall were Anglo-Saxon settlers, primarily in the north east of Cornwall between the Ottery and Tamar rivers, and in the lower Tamar valley, from around the 10th century onwards. There are a number of relatively early place names of English origin, especially in those areas.

The further spread of the English language in Cornwall was slowed by the change to Norman French as the main language of administration after the Norman Conquest. In addition, continued communication with Brittany, where the closely related Breton language was spoken, tended to favour the continued use of the Cornish language.

But from around the 13th to 14th centuries the use of English for administration was revived, and a vernacular Middle English literary tradition developed. These were probable reasons for the increased use of the English language in Cornwall. In the Tudor period, various circumstances, including the imposition of an English language prayer book in 1549, and the lack of a Cornish translation of any part of the Bible, led to a language shift from Cornish to English.

The language shift to English occurred much later in Cornwall than in other areas: in most of Devon and beyond, the Celtic language had probably died out before the Norman Conquest. However the Celtic language survived much later in the westernmost areas of Cornwall, where there were still speakers as late as the 18th century. For this reason, there are important differences between the Anglo-Cornish dialect and other West Country dialects.

Cornish was the most widely spoken language west of the River Tamar until around the mid-14th century, when Middle English began to be adopted as a common language of the Cornish people. As late as 1542 Andrew Boorde, an English traveller, physician and writer, wrote that in Cornwall were two languages, "Cornysshe" and "Englysshe", but that "there may be many men and women" in Cornwall who could not understand English. Since the Norman language was the mother tongue of most of the English aristocracy, Cornish was used as a lingua franca, particularly in the far west of Cornwall. Many Cornish landed gentry chose mottos in the Cornish language for their coats of arms, highlighting its high social status. The Carminow family used the motto "Cala rag whethlow", for example. However, in 1549 and following the English Reformation, King Edward VI of England commanded that the Book of Common Prayer, an Anglican liturgical text in the English language, should be introduced to all churches in his kingdom, meaning that Latin and Celtic customs and services should be discontinued. The Prayer Book Rebellion was a militant revolt in Cornwall and parts of neighbouring Devon against the Act of Uniformity 1549, which outlawed all languages from church services apart from English, and is cited as a testament to the affection and loyalty the Cornish people held for the Cornish language. In the rebellion, separate risings occurred simultaneously at Bodmin in Cornwall, and Sampford Courtenay in Devon—which would converge at Exeter, laying siege to the region's largest Protestant city. However, the rebellion was suppressed, due largely to the aid of foreign mercenaries, in a series of battles in which "hundreds were killed", effectively ending Cornish as the common language of the Cornish people. The Anglicanism of the Reformation served as a vehicle for Anglicisation in Cornwall; Protestantism had a lasting cultural effect upon the Cornish by way of linking Cornwall more closely with England, while lessening political and linguistic ties with the Bretons of Brittany.

The English Civil War, a series of armed conflicts and political machinations between Parliamentarians and Royalists, polarised the populations of England and Wales. However, Cornwall in the English Civil War was a staunchly Royalist enclave, an "important focus of support for the Royalist cause". Cornish soldiers were used as scouts and spies during the war, for their language was not understood by English Parliamentarians. Following the war there was a further shift to the English language by the Cornish people, which encouraged an influx of English people to Cornwall. By the mid-17th century the use of Cornish had retreated far enough west to prompt concern and investigation by antiquarians, such as William Scawen who had been an officer during the Civil War. As the Cornish language diminished, the people of Cornwall underwent a process of English enculturation and assimilation, becoming "absorbed into the mainstream of English life".

Large-scale 19th and 20th century emigrations of Cornish people meant that substantial populations of Anglo-Cornish speakers were established in parts of North America, Australia, and South Africa. This Cornish diaspora has continued to use Anglo-Cornish, and certain phrases and terms have moved into common parlance in some of those countries.