Brithenig is constructed language that was made up by Andrew Smith from New Zealand in 1996. The idea behind it is the question what the Welsh language would have looked like if it had been a Romance instead of a Celtic language: a sister language to French, Spanish and Italian, but one that looks much like Welsh. It plays a key-role in the made-up world of Ill Bethisad, where Latin took the place of Old Celtic as the spoken language of the people in Great Britain, thus giving way to the development of Brithenig.
Brithenig is the best known example of the so-called group of "alternative languages", languages that could have existed if history had been different. Another well-known example is Wenedyk, which does the same with Polish as Brithenig does with Welsh.