ABSTRACT: This study discusses paleoceanographic changes in the Japan Sea across the Miocene/Pliocene boundary based on foraminiferal distributions and geochemical evidence obtained in the Akita Prefecture of northern Japan.Q-mode cluster analysis differentiates six foraminiferal associations from a total of 55 assemblages. Their paleoceanographic implications are based on previous studies on the ecology of benthic foraminifers Spirosigmoilinella compressa and Miliammina echigoensis, microhabitat preferences of constituent species inferred from morphotypes, and other ecologic information of extant species. The stratigraphic distribution of each association reveals a history of oxygen depletion in the bottomwaters of the Japan Sea during the Late Miocene, followed by the oxic bottomwaterwith conditions of higher organic matter flux in the Early Pliocene. This interpretation is reinforced by the geochemical data of organic facies such as total organic carbon and hydrogen index fromRock-Eval analysis. It is inferred that this paleoceanographic change was a result of the rise in the eustatic sea level during the earliest Pliocene. Throughout the stratigraphic interval organic matter flux, related to primary productivity, is inferred to be higher in the northern sections than in the southern sections. This geographic variation is in agreementwith previous studies on other Japan Sea regions, and suggests that eutrophic conditions were prevalent in the northern region, where the Tsugaru Strait links the Japan Sea with the Pacific Ocean. The organic matter fluxes and oxygen depletion controlled by eustatic sea level variation are closely related to the source rock deposition of the oil and gas fields in the Japan Sea region.