From the Welsh Brycheiniog, one of the medieval kingdoms of Wales. Named after Bychan Brycheiniog (born 419), the Irish chieftain who traditionally established a kingdom there sometime in the 5th century.

Brecknock (pronounced phonetically) was the nearest the English could get to pronouncing Brycheiniog. Later shortened to Brecon when it became fashionable amongst the English not to pronounce consonants. (In the same way that, for example Brighthelmstone became Brighton)