The Japan Sea is directly influenced by the Asian monsoon, a system that transports moisture and heat across southeast Asia during the boreal summer, and is a major driver of the Earth's ocean‐atmospheric circulation. Foraminiferal and facies analyses of a 460 kyr record from IODP Expedition 346 Site U1427 in the Japan Sea reveal a record of nutrient flux and oxygenation that varied due to sea level and East Asian monsoon intensity. The East Asian summer monsoon (EASM) was most intense during MIS (Marine Isotope Stage) 5e, 7e, 9e and 11c when the Tsushima Warm Current flowed into an unrestricted well mixed normal salinity Japan Sea. Whereas East Asian winter monsoon (EAWM) conditions dominated MIS 2, 4, 6 and 8 when sea level minima restricted the Japan Sea resulting in low salinity and oxygen conditions in the absence of Tsushima flow. Reduced oxygen stratified, low salinity, higher productivity oceanic conditions characterise terminations TV, TIII, TII and TI when East China Sea Coastal Waters breached the Tsushima Strait. Chinese loess, cave and Lake Biwa (Japan) and U1427 proxy records suggests EASM intensification during low to high insolation transitions whereas the strongest EAWM prevailed during lowest insolation periods or high to low insolation transitions. Icesheet/CO2 forcing lead to the strongest EAWM events in glacials and enhanced EASM in interglacials. Mismatches between proxy patterns suggests latitudinal and land/sea thermal contrasts played a role in East Asian monsoon variability suggesting a complex interplay between ice sheet dynamics, insolation and thermal gradients controls monsoonal intensity.