Southwestern Brythonic (also known as SW Brittonic) is the name given to the now extinct Brythonic Celtic language which was spoken in what is now South West England (or the Westcountry) from the sixth century until the ninth (approximate dates).

After the Saxon advances split the British Celts of the South West peninsular from those of what is now Wales (following the Battle of Deorham in 577CE).

Southwestern Brythonic has little surviving direct written material to support it, but is is evidenced in placenames (such as the late form "Dunchideock" in Devon), in inscriptions, and from the Cornish and Breton languages which it is believed both derived from this common parent.

It has recently been the subject of a minor revival in Devon following the self-publication of a booklet entitled "A Handbook of West Country Brythonic: The Forgotten Celtic Tongue of South West England C.700 A.D. (Old Devonian)" by welsh linguist and publisher Joseph Biddulph, though the merits and accuracy of Biddulph's attempted reconstruction of the language are disputed.