A new species of capsalid monogenean, Benedenia armata n. sp., is described from Lethrinus haematopterus Temminck & Schlegel (Perciformes: Lethrinidae) from off Danjo Islands, Nagasaki Prefecture, Japan. The new species differs from other species of Benedenia Diesing, 1858 in that the common genital atrium has spines and the vas deferens has two swellings. Lethrinus haematopterus has been considered as the type-host of Benedenia ishikawae (Goto, 1894) based on the local name 'Kuchibi-dai'. However, this name refers not only to L. haematopterus but also to L. nebulosus (Forsskål), and L. haematopterus examined in this study was not infected by B. ishikawae. Further study is required to determine the type-host of B. ishikawae and to redetermine the species of Benedenia that are parasitic on Lethrinus spp. in the type-locality of B. ishikawae (Hagi, Yamaguchi Prefecture, Japan).