The Indo-West Pacific (IWP) benthos, which shows the highest diversity of marine organisms worldwide, is thought to have originated from the Tethys Ocean. Little is known about Miocene ostracodes in the IWP, that is, subsequent to the closure of the Tethys Ocean. To understand the paleobiogeography of Miocene ostracodes in the IWP, we examined ostracodes from the Middle Miocene—Lower Pliocene Osaki Formation, Kukinaga Group, at Tanegashima, Southwest Japan, revealing the presence of Neocytheromorpha, an IWP taxon that originated from South Australia. Neocytheromorpha occurs abundantly with Trachyleberis in Middle Miocene sublittoral deposits. This finding indicates that Neocytheromorpha migrated northward from South Australia, along the Australian coast, and settled in coastal areas of the West Pacific during the Oligocene—Middle Miocene. Because the migration coincided with diversification in the West Pacific and the invasion of corals and molluscs, it was synchronous with changes in the populations of the IWP biota. Two new species are systematically described: Acanthocythereis inouei sp. nov. and Neocytheromorpha priscipacifica sp. nov.