The Brittonic languages (also Brythonic or British Celtic; Welsh: ieithoedd Brythonaidd/Prydeinig; Cornish: yethow brythonek/predennek; Breton: yezhoù predenek) form one of the two branches of the Insular Celtic language family; the other is Goidelic. The name Brythonic was derived by Welsh Celticist John Rhys from the Welsh word Brython, meaning Ancient Britons as opposed to an Anglo-Saxon or Gael.

The Brittonic languages derive from the Common Brittonic language, spoken throughout Great Britain during the Iron Age and Roman period. In the 5th and 6th centuries emigrating Britons also took Brittonic speech to the continent, most significantly in Brittany and Britonia. During the next few centuries the language began to split into several dialects, eventually evolving into Welsh, Cornish, Breton, Cumbric, and probably Pictish. Welsh and Breton continue to be spoken as native languages, while a revival in Cornish has led to an increase in speakers of that language. Cumbric and Pictish are extinct, having been replaced by Goidelic and Anglic speech. The Isle of Man and Orkney may also have originally spoken a Brittonic language, but this was later supplanted by Goidelic on the Isle of Man and Norse on Orkney. There is also a community of Brittonic language speakers in Y Wladfa (the Welsh settlement in Patagonia).

Brittonic or Brythonic may refer to:

• Common Brittonic, or Brythonic, the Celtic language anciently spoken in Great Britain
• Brittonic languages, a branch of the Celtic languages descended from Common Brittonic
• Britons (Celtic people), or Celtic Britons, the Celtic people of Great Britain in ancient times