Glacial isolation of the Japan Sea results in poor carbonate preservation and unusually low oxygen isotope (d18O) ratios from low-saline/low-d18O waters accumulating in the basin. Using the sediments of Integrated Ocean Drilling Program (IODP) Site U1427, a shallow-water site in the southern Japan Sea, we present a continuous foraminiferal (Uvigerina spp.-based) d18O record encompassing the mid-Pleistocene transition (MPT). Using Uvigerina spp. Mg/Ca ratios and a new core-top Mg/Ca-temperature calibration for the Japan Sea (see supplementary material to main manuscript) it is revealed that the d18O record shows the Japan Sea-typical low glacial d18O values in the late phase of the MPT, across Marine Isotope Stages (MIS) 24-17, while earlier MPT glacials, across MIS 39-25, are characterised by high d18O values. We propose that high glacial d18O values are the result of an improved connection between the shallow, southern Japan Sea and adjacent seas during early MPT glacials. The impact of this palaeoceanographic mode, if continued to deep-water sites, the effects of it would make the interpretation of dark/light sediment layers as glacial/interglacial deposits uncertain.