The monogenean Neobenedenia girellae (Hargis, 1955) Yamaguti, 1963 is redescribed and reported for the first time in Japan. The parasite was recovered from the body surface, fins, and occasionally from the eyes of 14 species, comprising 5 families of cultured marine fishes from several localities in southwestern Japan. Neobenedenia melleni (MacCallum, 1927) sensu Kaneko et al. (1988) from tilapia (Oreochromis mossambicus) in Hawaii is synonymized with this species. Examination of original specimens (syntypes) of N. melleni sensu MacCallum (1927) revealed differences with N. girellae in having a wide and rounded body, a prominently large anterior hamuli, and absence of glands of Goto. This Neobenedenia from Japanese fishes sometimes showed an unusual morphology of the individual parts of the median sclerites. The potential threat of N. girellae to the health of cultured Japanese fishes is indicated by its low host specificity, wide distribution, and ability to cause mortality due to heavy infection. Unregulated importation of amberjack fry (Seriola dumerili) to Japan appears to be the source of N. girellae infection in Japanese fishes since 1991.