Abstract: The earliest known Paleogene mammals from Japan are reported from the Eocene Akasaki Formation, Amakusa area, Kumamoto Prefecture and from the stratigraphically equivalent Nakakoshiki Formation, Koshiki Islands, KagoshimaPrefecture, western Kyushu. Preliminary work has revealed a high diversity of mammals from these formations, including at least 18 different species from nine orders. The Akasaki fauna is represented by three trogosine tillodonts, two coryphodontid pantodonts, an isectolophid perissodactyl, two dichobunoid artiodactyls, a hyopsodontid condylarth, a sivaladapid primate, an unidentified insectivoran, and two possible ctenodactyloid rodents. The mammal fauna from the Nakakoshiki Formation includes a coryphodontid pantodont, two small brontotheriid perissodactyls, a hypsodont hyopsodontid condylarth, two rodents, and a miacid carnivoran. These Japanese mammal assemblages are from near the Early/Middle Eocene boundary, which is previously poorly known in Asia. Given the occurrences of trogosine tillodonts and brontotheriid perissodactyls, the Akasaki and Nakakoshiki faunas are younger than Bumbanian faunas and are most likely correlated with Arshantan faunas on the Asian mainland. The Japanese faunas contain several new taxa unknown from the Asian mainland, and there are unique faunal combinations in the mammal assemblages.