A long, tubular “parasite” was found in the muscle of greater amberjack Seriola dumerili cultured in Kagoshima and Miyazaki prefectures. Infection with this parasite was very rare, only occurring in two amberjacks from Kagoshima and one from Miyazaki. Molecular analysis indicates the parasite is a cestode of the order Trypanorhyncha, most closely related to the subfamily Grillotiinae, family Lacistorhynchidae. The tubular structure was a blastocyst. There was no evidence of the scolex in the blastocysts, which may not have been sampled intact, making it impossible to identify the parasite. This is the first report of Trypanorhyncha infection in greater amberjack in Japan.