Leeches belonging to the genus Ozobranchus are ectoparasitic on sea and freshwater turtles. The freshwater species O. jantseanus has been recorded from China and Japan. Ozobranchus jantseanus inhabiting Japan is considered to be a non-indigenous species, because their primary host, the Reeves' pond turtle, Mauremys reevesii, is thought to have been introduced in the last few centuries from adjacent Asian countries. To assess whether the Japanese populations of O. jantseanus were likely to have been introohced, their genetic diversity was investigated using mitochondrial cytochrome c oxidase subunit I sequences. The very low sequence diversity as well as the historical record of this species from Japan suggest that Japanese populations of O. jantseanus may have been artificially introduced along with their host turtles. Molecular phylogenies of COI showed that two marine Ozobranchus species formed a Glade together with the freshwater O. jantseanus.