A 100–400 km wide region of the coastal Eastern Arabian Sea (EAS), off the west-coast of India, is characterized by a low-salinity tongue formed by the inflow of low-salinity surface water from the Bay of Bengal (BoB). This low-salinity tongue is largely driven by the sea level higher in BoB than in the Arabian Sea and by alongshore pressure gradient between southern- and northern-EAS, and is expected to respond to summer monsoon freshwater flux to the bay. Here, we report past variation in the relative intensity of summer- and winter-monsoons based on changes in the north–south salinity gradient within this low-salinity tongue. The salinity gradient is estimated from paired measurement of delta¹⁸O and Mg/Ca in Globigerinoides sacculifer extracted from sediment cores collected at northern high-salinity and southern low-salinity locations within this tongue. The Last Glacial Maximum (LGM) to peak-Holocene delta¹⁸O and sea surface temperature gradients at both locations are ~ -2‰ and + 2 °C respectively, while the sea surface salinity gradient at northern-EAS is 0.5 psu higher than in the southern-EAS, suggesting distinctly different SSS structure in the LGM-EAS. The north–south surface salinity gradient was also larger by ~ 0.5 psu during the LGM (1.2 psu) as compared to the gradient during the Holocene (0.7 psu). Increased north–south surface salinity gradient during the LGM suggests diminished flow of low-salinity water into the coastal EAS caused by combined effect of decreased freshening of the BoB and reduced seasonal mountain-river discharge into the EAS. Such surface hydrographic conditions in the coastal EAS clearly indicate significantly weakened summer monsoons and strengthened winter monsoons during the LGM.