A Breton lai, also known as a narrative lay or simply a lay, is a form of medieval French and English romance literature. Lais are short (typically 600-1000 lines), rhymed tales of love and chivalry, often involving supernatural and fairy-world Celtic motifs.

The earliest Breton lais to survive in writing are probably The Lais of Marie de France, thought to have been composed in the 1170s by Marie de France, a French poet living in England in the late 12th and early 13th century. From descriptions in Marie's lais, and in several anonymous Old French lais of the 13th century, we know of earlier lais of Celtic origin, perhaps more lyrical in style, sung by Breton minstrels. It is believed that these Breton lyric lais, none of which has survived, were introduced by a summary narrative setting the scene for a song, and that these summaries became the basis for the narrative lais. 'The Lay of the Beach', one of around twenty Old French lais translated into Old Norwegian prose in the 13th century, gives a detailed description of William the Conqueror's commissioning of what appears to be a lyric lai to commemorate a period spent at Barfleur.

The earliest written Breton lais were composed in a variety of Old French dialects, and some half dozen lais are known to have been composed in Middle English in the 13th and 14th centuries by various English authors. These include:

• Sir Orfeo, 'Sir Degaré', 'Sir Gowther', 'Emaré' and 'The Erle of Tolouse', all by anonymous authors
• Lay le Freine, a translation of Marie de France's 'Le Fresne'
• The Franklin's Tale from the Canterbury Tales by Geoffrey Chaucer. (The Wife of Bath's Tale is sometimes identified as a Breton lai, but in fact it contains none of the generally-agreed defining features of the genre.)
• Sir Launfal, by Thomas Chestre (a retelling of an earlier Middle English lai, 'Landavale', itself a translation of Marie de France's 'Lanval')
• Lai du Cor, by Robert Biket