Lyonesse or Lyonnesse is the sunken land believed in legend to lie off the Isles of Scilly, to the south-west of Cornwall. It is sometimes associated with Avalon.

According to Arthurian legend, one of the signs of King Arthur's return will be that Lyonesse will rise from the depths again.

Alfred, Lord Tennyson's Arthurian epic, Idylls of the King, describes Lyonesse as the site of the final battle between Arthur and Modred. One passage in particular references legends of Lyonesse as a land fated to sink beneath the ocean:

Then rose the King and moved his host by night,
And ever pushed Sir Modred, league by league,
Back to the sunset bound of Lyonesse --
A land of old upheaven from the abyss
By fire, to sink into the abyss again;
Where fragments of forgotten peoples dwelt,
And the long mountains ended in a coast
Of ever-shifting sand, and far away
The phantom circle of a moaning sea.

A real-life counterpart to Lyonesse is the fishing port of Dunwich.

Lyonesse in fiction

It has sometimes been used as the setting for fantasy stories, notably Jack Vance's Lyonesse trilogy. In Stephen R. Lawhead's Pendragon Cycle, Lyonesse is where refugees from Atlantis (the 'Fair Folk') settle; 'Lyonesse' being derived from the Celtic corruption of 'Atlantis.' See also: fantasy world.