Lyonesse (/liːɒˈnɛs/ lee-uh-NESS) is a kingdom which, according to legend, consisted of a long strand of land stretching from Land's End at the southwestern tip of Cornwall, England, to what is now the Isles of Scilly in the Celtic Sea portion of the Atlantic Ocean. It was considered lost after being swallowed by the ocean in a single night. The people of Lyonesse were said to live in fair towns, with over 140 churches, and work in fertile, low-lying plains. Lyonesse's most significant attraction was a castle-like cathedral that was presumably^{[by whom?]} built on top of what is now the Seven Stones Reef between Land's End and the Isles of Scilly, some 18 miles (29 km) west of Land's End and 8 miles (13 km) north-east of the Isles of Scilly. It is sometimes spelled Lionesse.

Lyonesse is mentioned in Arthurian legend, specifically in the tragic love-and-loss story of Tristan and Iseult. It was the home of the hero Tristan (one of the Knights of the Round Table), whose father Meliodas was king of Lyonesse. After the death of Meliodas, Tristan became the heir of Lyonesse, but he was never to take up his inheritance because the land sank beneath the sea while he was away at his uncle King Mark's court in Cornwall. In later traditions, Lyonesse is said to have sunk beneath the waves in a single night, but stories differ as to whether this catastrophic event occurred on 11 November 1099, or 10 years earlier. According to one legend, the people of Lyonesse had committed a crime so terrible that God took his revenge against them and their kingdom. The exact nature of the crime is never specified, but the legend tells of a horrific storm that occurred over the course of a single night, resulting in an enormous wave that swallowed the kingdom.

Local Cornwall village tourism guides offer stories of a man who escaped the storm and a subsequent wave while riding a white horse. Apparently, the horse lost one of its shoes during the escape. The rider's name is thought to be Trevelyan (or Trevilian). The rider had been out hunting during the day and had fallen asleep under a tree. Trevelyan was awoken by a horrible noise and raced across the land to higher ground. This story is linked to local Cornish families who have used the image of three horseshoes as part of their armorial bearings for generations. One family in particular goes by the name Vyvyan, and is one of Cornwall's oldest families; they also have a crest of a white horse and claim to be descendants of the sole survivor, Trevelyan. The Vyvyan family claims that Trevelyan was the last governor of the lost kingdom before Lyonesse was swallowed by the ocean.

Today, many myths and legends continue to arise about Lyonesse without physical evidence. Included among these legends are tales of local fishermen who claim that on calm days, one can still hear the bells of the many churches softly ringing in the seas off the west Cornish coast. Local fishermen also claim that they have caught glass, forks, and wood in their fishing nets.

A 2009–2013 joint study titled The Lyonesse Project: A Study of the Coastal and Marine Environment of the Isles of Scilly was commissioned by English Heritage and carried out by the Historic Environment Projects, Cornwall Council, with a team of academics, local experts, and enthusiasts "to reconstruct the evolution of the physical environment of the Isles of Scilly during the Holocene, the progressive occupation of this changing coastal landscape by early peoples, and their response to marine inundation and changing marine resource availability."

The project found that much of the story of Lyonesse can be "dismissed as fantasy"; legends and memories of submergences are common throughout coastal northwestern Europe. However, the study did conclude that the Isles of Scilly were once a single large island, separated into smaller islands by the rapid rise of the sea. Stone walls have been found under the water in the vicinity of the Isles of Scilly, which support the findings that sea level rises affected the towns of the area, although whether they are evidence of buildings or the remains of medieval fish traps remains unclear.

In medieval Arthurian legend, no references are made to the sinking of Lyonesse, because the name originally referred to a still-existing place. Lyonesse is an English alteration of French Léoneis or Léonois (earlier Loönois), a development of Lodonesia, the Latin name for Lothian in Scotland. Continental writers of Arthurian romances were often puzzled by the internal geography of Great Britain;^{[dubious – discuss]} thus it is that the author of the French Prose Tristan appears to place Léonois beside Cornwall.

In English adaptations of the French tales, Léonois, now "Lyonesse", becomes a kingdom wholly distinct from Lothian, and closely associated with the Cornish region, though its exact geographical location remained unspecified. The name was not attached to Cornish legends of lost coastal lands until the reign of Elizabeth I of England. However, the legendary lost land between Land's End and Scilly has a distinct Cornish name: Lethowsow. This derives from the Cornish name for the Seven Stones Reef, on the reputed site of the lost land's capital and the site of the notorious wreck of the Torrey Canyon. The name means 'the milky ones', from the constant white water surrounding the reef.^{[citation needed]}