Lyonesse is a country in Arthurian legend, particularly in the story of Tristan and Iseult. Said to border Cornwall, it is most notable as the home of the hero Tristan, whose father was king. In later traditions Lyonesse is said to have sunk beneath the waves some time after the Tristan stories take place, making it similar to Ys and other lost lands in medieval Celtic tales, and perhaps connecting it with the Isles of Scilly.

In medieval Arthurian legend, there are no references to the sinking of Lyonesse, for the simple reason that 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; thus it is that the author French Prose Tristan appears to place Léonois contiguous, by land, to 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.. However, the legendary lost land between Land's End and Scilly has a distinct Cornish name: Lethesow (sometimes rendered 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 famous wreck of the Torrey Canyon. The name translates into English as "the milky ones", from the constant white water surrounding the reef.