Plectanocotyle major sp. n. (Monogenea: Polyopisthocotylea: Plectanocotylidae) is described from Chelidonichthys obscurus (Bloch et Schneider, 1801) collected from the Mediterranean coasts (Tunisia and France). The new species is compared to Plectanocotyle gurnardi collected on Chelidonichthys gurnardus and the only other valid species in the genus Plectanocotyle. The new species differs from P. gurnardi by the clamp size (137 µm vs 64 µm for clamp length and 110 µm vs 60 µm for clamp width, respectively) and the length of the male copulatory organ (107 µm vs 144 µm, respectively). Species of the Plectanocotylidae seem to be characterized by an oioxenic specificity.