Cornish mythology is the folk tradition and mythology of the Cornish people. It consists partly of folk traditions developed in Cornwall and partly of traditions developed by Britons elsewhere before the end of the first millennium, often shared with those of the Breton and Welsh peoples. Some of this contains remnants of the mythology of pre-Christian Britain.

The folklore of Cornwall often consists of tales of giants, mermaids, Bucca, piskies or the 'pobel vean' (little folk.) These tales are still popular today, with some events hosting a 'droll teller' or storyteller,^{[page needed]} to share Cornish myths and legends. The myths and stories of Cornwall have found much publishing success, particularly in children's books. The fairy tale Jack the Giant Killer takes place in Cornwall. Many early British legends associate King Arthur with Cornwall, putting his birthplace at Tintagel, the court of King Mark of Cornwall, uncle of Tristan and husband of Iseult, the most famous Cornish lovers.

Cornwall shares its ancient cultural heritage with its 'Brythonic cousins' Brittany and Wales, as well as Ireland and parts of England such as neighbouring Devon. Many ancient tales of the Bards, whether the Arthurian Cycle, Tristan and Iseult, or the Mabinogion take place in the ancient kingdom of Cerniw between Greater and Lesser Britains with a foot on either side of the 'British Sea' Mor Brettanek/Mor Breizh.

Part of Cornish mythology is derived from tales of seafaring pirates and smugglers who thrived in and around Cornwall from the early modern period through to the 19th century. Cornish pirates exploited both their knowledge of the Cornish coast as well as its sheltered creeks and hidden anchorages. For many fishing villages, loot and contraband provided by pirates supported a strong and secretive underground economy in Cornwall.^{[page needed]}

Legendary creatures that appear in Cornish folklore include buccas, knockers, Giants, and Pixies. Tales of these creatures are thought to have developed as supernatural explanations for the frequent and deadly cave-ins that occurred during 18th century Cornish tin mining, or else a creation of the oxygen-starved minds of exhausted miners who returned from the underground.

The knocker is said to be about two feet tall and grizzled, but not misshapen. They tend to live underground. Here they wear tiny versions of standard miner's garb and commit random mischief, such as stealing a miner's unattended tools and food. They were often cast a small offering of food – usually the crust of a pasty – to appease their malevolence.

Many landscape features, from the barren granite rock features on Bodmin Moor, to the dramatic cliff seascape, to the mystical form of St Michael's Mount are explained as the work of Giants and English tales such as the early eighteenth century Jack the Giant Killer may recall much older British folk traditions recorded elsewhere in medieval Welsh language manuscripts and closely related to the folk traditions of Dartmoor in neighbouring Devon.

There is a Cornish legend of the lost land of Lyonesse, supposedly lost to the sea in one night. It is claimed to represent the folk memory of the flooding of the Isles of Scilly and Mount's Bay near Penzance. For example, the Cornish name of St Michael's Mount is Karrek Loos y'n Koos, literally, "the grey rock in the wood". The Breton legend of Ys is a similar concept.