Cornish nationalism is an umbrella term that refers to a cultural, political and social movement based in Cornwall, the most southwestern part of the island of Great Britain, which has for centuries been administered as part of England, within the United Kingdom. It is usually based on three general arguments:

Cornish nationalists generally seek some form of autonomy for Cornwall. Some advocate a Cornish Assembly, a regionally elected legislature that would make Cornwall a region of the United Kingdom, aiming to bring the current unitary council and related institutions into similar political and administrative status as Scotland, Wales and Northern Ireland, whilst others advocate going further by recognising the Crown Dependency status of the Duchy and implementing equivalent arrangements.

Many supporters will, in addition to making legal or constitutional arguments, stress that the Cornish are a distinct ethnic group or nation, that people in Cornwall typically refer to 'England' as beginning east of the River Tamar, and that there is a Cornish language. If correct they argue the Cornish therefore have a right to national self determination.