Breton (, French: [bʁətɔ̃]; brezhoneg [bʁeˈzɔ̃ːnɛk] (listen) or [brəhɔ̃ˈnek] in Morbihan) is a Southwestern Brittonic language of the Celtic language family spoken in Brittany, part of modern-day France. It is the only Celtic language still in use on the European mainland, albeit as a member of the insular branch instead of the continental grouping.

Breton was brought from Great Britain to Armorica (the ancient name for the coastal region that includes the Brittany peninsula) by migrating Britons during the Early Middle Ages, making it an Insular Celtic language. Breton is most closely related to Cornish, another Southwestern Brittonic language. Welsh and the extinct Cumbric, both Western Brittonic languages, are more distantly related.

Having declined from more than one million speakers around 1950 to about 200,000 in the first decade of the 21st century, Breton is classified as "severely endangered" by the UNESCO Atlas of the World's Languages in Danger. However, the number of children attending bilingual classes rose 33% between 2006 and 2012 to 14,709.