The Japan Sea is connected with the Okhotsk Sea and Northwest Pacific through three straits, viz. Mamiya (Tartarskiy), Soya and Tsugaru, and with the East China Sea through Tsushima Strait, all having sill depths of <140 m. Therefore, the Japan Sea was almost isolated from the surrounding seas during the Last Glacial Maximum (LGM; ca. 20 ka). As a result of nearly broken communication with the surrounding seas and strengthened surface stratification due to locked fresh water in the basin during the LGM, it became an anoxic basin. This major shift in the surface water properties of the basin left its imprint in the oxygen isotopic composition (δ18OPF) of the planktonic foraminifera. The δ18OPF exhibits extremely low values (∼0.5‰ vs. Belemnite from Pedee formation, South Carolina: PDB) during the last two glacial maxima, ca. 20 ka (Marine Isotope Stage (MIS) 2.2) and ca. 140 ka (MIS 6.2). These decreases are in contrast to the expected generally high glacial values of >3‰ recorded in the MIS 3–4 sections of the studied cores. This observation indicates nearly similar surface water conditions in the Japan Sea during both the previous glacial maxima, and hence suggests negligible effect of strait tectonics on the sea level since the last 140 ka. Low δ18OPF values with small amplitude are also observed at the cold stage MIS 6.4, but not observed at another cold stage MIS 4.2. This observation suggests that the low-salinity surface water in the Japan Sea started to develop at the sea level between MIS 4.2 and 6.4. Judging from the cross-section of the Tsushima Strait and the sea floor topography of the East China Sea, the sea level between MIS 4.2 and 6.4 must have been between 90 m and 100 m below the present. From a proportional relationship between the standard δ18O curve (LR04) in deep-sea cores and globally averaged sea-level change, the eustatic sea level obtained for the LGM is 120 ± 7 m below the present. Thus the Japan Sea, in a far-field location from the continental ice sheets, provides an important clue for estimation of the eustatic sea level at the LGM. Copyright © 2012 John Wiley & Sons, Ltd.