The Brittonic languages (also Brythonic or British Celtic; Welsh: ieithoedd Brythonaidd/Prydeinig; Cornish: yethow brythonek/predennek; and Breton: yezhoù predenek) form one of the two branches of the Insular Celtic languages; the other is Goidelic. It comprises the extant languages Breton, Cornish, and Welsh. The name Brythonic was derived by Welsh Celticist John Rhys from the Welsh word Brython, denoting a Celtic Briton as distinguished from Anglo-Saxons or Gaels.

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, in much of Britain the language was replaced by Old English and Scottish Gaelic, with the remaining Common Brittonic language splitting into regional 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 Cumbric and Pictish are extinct, having been replaced by Goidelic and Anglic speech. There is also a community of Brittonic language speakers in Y Wladfa (the Welsh settlement in Patagonia).

The names "Brittonic" and "Brythonic" are scholarly conventions referring to the Celtic languages of Britain and to the ancestral language they originated from, designated Common Brittonic, in contrast to the Goidelic languages originating in Ireland. Both were created in the 19th century to avoid the ambiguity of earlier terms such as "British" and "Cymric". "Brythonic" was coined in 1879 by the Celticist John Rhys from the Welsh word Brython. "Brittonic", derived from "Briton" and also earlier spelled "Britonic" and "Britonnic", emerged later in the 19th century. "Brittonic" became more prominent through the 20th century, and was used in Kenneth H. Jackson's highly influential 1953 work on the topic, Language and History in Early Britain. Jackson noted by that time that "Brythonic" had become a dated term: "of late there has been an increasing tendency to use Brittonic instead." Today, "Brittonic" often replaces "Brythonic" in the literature. Rudolf Thurneysen used "Britannic" in his influential A Grammar of Old Irish, although this never became popular among subsequent scholars.

Comparable historical terms include the Medieval Latin lingua Britannica and sermo Britannicus and the Welsh Brythoneg. Some writers use "British" for the language and its descendants, although, due to the risk of confusion, others avoid it or use it only in a restricted sense. Jackson, and later John T. Koch, use "British" only for the early phase of the Common Brittonic language.

Before Jackson's work, "Brittonic" and "Brythonic" were often used for all the P-Celtic languages, including not just the varieties in Britain but those Continental Celtic languages that similarly experienced the evolution of the Proto-Celtic language element /kʷ/ to /p/. However, subsequent writers have tended to follow Jackson's scheme, rendering this use obsolete.

The name "Britain" itself comes from Latin: Britannia~Brittania, via Old French Bretaigne and Middle English Breteyne, possibly influenced by Old English Bryten[lond], probably also from Latin Brittania, ultimately an adaptation of the native word for the island, *Pritanī.

An early written reference to the British Isles may derive from the works of the Greek explorer Pytheas of Massalia; later Greek writers such as Diodorus of Sicily and Strabo who quote Pytheas' use of variants such as πρεττανική (Prettanikē), "The Britannic [land, island]", and νησοι βρεττανιαι (nēsoi brettaniai), "Britannic islands", with Pretani being a Celtic word that might mean 'painted ones' or 'tattooed folk', referring to body decoration.

Knowledge of the Brittonic languages comes from a variety of sources. The early language's information is obtained from coins, inscriptions, and comments by classical writers as well as place names and personal names recorded by them. For later languages, there is information from medieval writers and modern native speakers, together with place names. The names recorded in the Roman period are given in Rivet and Smith.