A new nematode species, Capillaria acanthopagri n. sp. (Capillariidae), is described from the intestine of the marine fish (black porgy) Acanthopagrus schlegelii schlegelii (Bleeker) from coastal waters of the western north Pacific Ocean off Kochi, Shikoku Island, Japan. The new species, belonging to the subgenus Neocapillaria Moravec, 1987, differs from other congeneric species of this subgenus from marine fishes (with the exception of C. navonae Timi, Rossin and Lanfranchi, 2006) mainly in the length (204–285 µm), shape, and structure of the spicule; and except for C. cooperi Johnston and Mawson, 1945, in markedly small body measurements (males and females 3–4 and 6–10 mm long, respectively). It is characterized by the presence of 33–43 elongate stichocytes; a heavily sclerotized spicule with rough transverse grooves; a spinous spicular sheath; and by eggs measuring 27–30 × 57–60 µm, without protruding polar plugs. Capillaria acanthopagri n. sp. is the first known species of Neocapillaria parasitizing fishes of the perciform Sparidae and the second species of this subgenus recorded from fishes of the Pacific Ocean.