A trematode identified macroscopically as Hirudinella sp. was found free in the body cavity of a young individual (197 mm standard length) of Pacific bluefin tuna, Thunnus orientalis (Temminck and Schlegel, 1844), cultured in coastal waters of the Bungo Strait (western North Pacific Ocean) off Oita Prefecture, Kyushu, western Japan. When found, the trematode was pale purple in color and as big as 34 mm long. This finding represents the first record of Hirudinella sp. from T. orientalis. The infected fish showed no external disease sign, but part of the surface of its visceral organs around the trematode was black-colored. The fish probably had become infected by the trematode in the ocean before it was farmed.