Two big trematodes, provisionally identified as a hirudinellid, were found individually in the muscle of two juveniles (ca. 150 mm fork length) of Japanese amberjack, Seriola quinqueradiata Temminck and Schlegel, 1845, cultured in mid-May 2015 in coastal waters of Tosa Bay off Kochi Prefecture, Shikoku, western Japan. The trematodes were 7-15 mm long in fresh conditions. The juveniles were caught in the western North Pacific Ocean off Kochi Prefecture around 10 April 2015 and then cultured in net pens. The body surface of the infected juveniles was uneven in the affected area, and black substances, probably excreted by the trematodes, were found in the host’s muscle.