A new anisakid nematode, Hysterothylacium japonicum sp. n. is described from a single female specimen collected from the stomach of the rare marine fish, Trachipterus ishikawai Jordan et Snyder, from the Sea of Japan (Toyama Bay). It is characterized mainly by the presence of broad cervical alae, shape and structure of lips, length of the caecum and ventricular appendix (11.43 and 6.35 mm, respectively) and their ratio (1:0.55), long and slender tail, and by unusually large body measurements (body length 181 mm). H. japonicum is the first species of adult ascaridoid nematodes reported from fishes of the order Lampridiformes.