Lyonesse is the land believed in legend to lie off the Scilly Isles, to the south-west of Cornwall.

It has sometimes been used as the setting for fantasy stories, notably Jack Vance's Lyonesse trilogy. See also: fantasy world.

An excerpt from an ancient book discovered in an old castle along the coast of Cornwall;

Heerke ye men, for heyre wonce wast thye guart lande of Lyonesse, nawe sonken oonter thye see. Guart foartreses wure bilt heyre, andd alle uf Angland and Newstria (Frankish name for Gaul) cayme toogethre. Bat withyn a guart stoarme, thyse landes wure washd' a'wae, andd alle uf thye landes uf Lyonesse wure takd'n bye the see.

It seems evident that this excerpt was written by monks much later, but evidence of a low-lying landbridge have been found. As written by ancient authors, it is believed that this land connected England and France, having maybe a sandy tidal wash in the center, where the two countries were eventually separated.