A new species of janiridean isopod, Austrofilius mediterraneus sp. nov., from the Columbretes Islands (Castellón de la Plana), Mediterranean coast of the Iberian Peninsula, is described, given it is the first record of the genus in the Northern Hemisphere. It is mainly distinguished from the other two species of the genus by the male pleopod 1, which is wider at the apex and with hooked lateral lobes, curved and nearly surpassing medial lobes. Furthermore, the female operculum shows only four distolateral setae. The rostrum of Austrofilius mediterraneus sp. nov. is extended into single frontolateral tips but is shorter than in A. furcatus Hodgson, 1910.