The Insular Celtic language hypothesis groups the Goidelic languages, which include Irish, Scottish Gaelic and the recently extinct Manx, together with the Brythonic languages, of which the modern ones are Welsh, Breton, and the moribund Cornish.

The nomenclature "Insular" refers to the location of the areas where these languages have been traditionally spoken, that being the British Isles (Latin insula - "island"). It therefore also refers to the notion that the Brythonic and Goidelic languages evolved together in those islands, having a common ancestor more recent than any shared with the Continental Celtic languages (Celtiberian, Gaulish and Lepontic among others, all of which are long extinct).

The proponents of the Insular Celtic hypothesis point to shared innovations among Insular Celtic languages, including inflected prepositions and VSO word order. They assert that an etymological partition that lumps the Brythonic languages and Gaulish (all P-Celtic) on one side and the Goidelic languages with Celtiberian (all Q-Celtic) on the other may be a superficial one, as the identical sound shift (Q to P) could have occurred at least twice, once as Celts crossed into the Iberian peninsula and at a different instance in Britain, in the predecessors of Celtiberian and Brythonic, respectively.