Acid-resistant organic microfossils, such as pollen and dinoflagellate cysts, are often well preserved and can be used as a tool for bioecostratigraphy. Therefore, organic microfossils recorded in the hemipelagic sediments can play an important role in reconstructing the paleoclimate and paleoceanography during deposition. A palynological study was performed using the International Ocean Discovery Program (IODP) Expedition 346 Site U1430 in the Eastern South Korea Plateau (ESKP) in the East Sea This study revealed the variability of paleoclimate and paleoceanography in the region during the late Miocene, indicating that the climate and geography of the East Sea changed dramatically over 11 Ma. During the mid-Miocene, a climate optimum period, the Northern Hemisphere was characterized by a free ice sheet in polar areas, with very warm SST. Unlike today, the northern (deep) and eastern (shallow) parts of the East Sea have been open up to the late Miocene. The intrusion of low-dissolved oxygen and nutrient-rich seawater from the northern and central strait would have caused ventilation in the East Sea and maintained high productivity. A large amount of organic matter settled on the seabed and decomposed, potentially consuming enough oxygen to produce anoxic seafloor conditions, or there is the possibility that amorphous organic matter (AOM) was well preserved under the influence of North Pacific Deep Water (NPDW) with low oxygen. After ca. 8.2 Ma, more oxygenated cold seawater under the late Miocene cooling climate condition had been flowed into the East Sea, becoming a suboxic environment where AOM could not be well preserved and increased the dinoflagellate proportion due to the decreased dilution effect as a low proportion of AOM. Subsequently during the early Pliocene, the warm temperate broad-leaved pollen and cool temperate conifer pollen dominated under a warm climate, and the high sea-surface temperature (SST) allowed the warm water dinoflagellate species to dominate their ecosystem. In addition, eutrophic dinoflagellate species has flowed into the East Sea through the northern strait from the nutrient-rich subarctic North Pacific. Subsequently, during the northern hemisphere glaciation period (NHG), the warm broad-leaved pollen decreased, and the boreal conifer pollen was sustained at high concentrations due to intensification of the East Asian winter monsoon (EAWM). Under the strengthened cold?dry climate, cold-water dinoflagellate species dominated the East Sea due to a decrease in SST. Moreover, oligotrophic dinoflagellates predominated because the nutrient-rich seawater inflow was weakened due to the local uplift of the North Strait of the East Sea. Unlike Central Asia, which sustained a cold?dry climate up to 2 Ma, in the East Sea there was a cold-moist climate because of the effect of a weak Tsushima Warm Current (TWC) intrusion during the NHG period caused by increased snowfall on the eastern coast of the Korean Peninsula. The TWC flowed into the East Sea after ca. 2 Ma because of the stretching of the Okinawa Trough and the deepening of the Korea Strait (KS). The glacial?interglacial climate has changed distinctly across the Mid-Pleistocene Transition (MPT). The EAWM intensified during the glacial period, and a relatively warm climate dominated during the interglacial period. A high proportion of low salinity-eutrophic dinoflagellates indicates that the East China Sea Coastal Water (ECSCW) intensified during the relatively warm interglacial period after the MPT. Moreover, after 0.5 Ma, the ECSCW flowing into the East Sea was controlled by eustatic sea-level fluctuation due to an increase in the amplitude of the glacial?interglacial cycle. Phytoclast in the form of plant debris in terrestrial sediments can be transported by water to distant areas because they are lighter than inorganic particles. Phytoclast concentration changes are generally negatively correlated with the global sea-level curve, and their anti-phase cycles with high amplitude are clearly evident during the last ca. 750 ka within the geotectonic stabilization period. In particular, several coarse-grained phytoclasts were observed during the glacial period, including the last glacial maximum (LGM). These findings suggest that the concentration and size of phytoclasts flowing into the East Sea were influenced by changes in the distance of the source area, depending on the water depth of the strait and nearby shelves, due to sea-level changes in tandem with glacial?interglacial cycles and geotectonic events.