A jaw fragment of a large mosasaurine (Squamata, Mosasauridae) was discovered from the uppermost Cretaceous Izumi Group in Sennan City, Osaka Prefecture, Southwest Japan as a concretionary float. While the specimen is fragmentary, three complete and one partial alveoli are preserved, the anteriormost one being largest. The third alveolus contains a replacement tooth crown, whose lingual surface was prepared and exposed. The relatively straight crown possesses a smooth enamel surface and two well developed carinae that are oriented in a fore-and-aft position. A deep, columnar root supports such a crown and, when combined with the crown features, suggests that the specimen has the closest taxonomic affinity with a deep-jawed, non-durophagous mosasaurine, Prognathodon. Beginning in the latest Campanian-early Maastrichtian, Prognathodon increased their body size and subsequently became globally distributed during the Maastrichtian, involving the invasion of the Pacific in as early as the late early Maastrichtian. Combined with a minimally estimated mandible length of 119.6 cm, the occurrence of the new specimen in the northwest Pacific suggests its Maastrichtian age, which is congruent with the age assignment to the Izumi Group along the Izumi Mountains on the basis of various macroinvertebrate correlations and magnetostratigraphic studies.