Paleoclimatic records from the Bay of Bengal are rare. We reconstruct the sea-surface temperature (SST) and salinity from paired δ¹⁸O and Mg/Ca measurements in planktonic foraminifera Globigerinoides ruber from the western Bay of Bengal core VM29-19. Our data suggest that SST and seawater δ¹⁸O (δ¹⁸O_sw) were ~3°C colder and ~0.6‰ depleted, respectively, during the Last Glacial Maximum (LGM) compared to the early Holocene. The most enriched δ¹⁸O_sw values were found between 18.2 and 15.6 ka interval. Depleted LGM δ¹⁸O_sw values suggest a wet climate which freshened the Bay of Bengal sea surface. Our data further indicate that the monsoon was stronger in the Bølling/Allerød and weaker in the Younger Dryas periods. The most depleted early Holocene δ¹⁸O_sw values suggest that the monsoon was stronger and wetter resulting in a humid climate. After ~5 ka the Indian summer monsoon weakened significantly, indicating less dilution of the sea surface by the Ganges-Brahmaputra-Meghna outflow and/or less direct rainfall. We hypothesize that the prevailing late Holocene dry climate may have caused the diminishment and subsequent abandonment of the settlements of the great Indus Valley Civilizations. Our Bay of Bengal climate records are consistent with those from the Andaman Sea, corroborating broad regional changes in the Indian summer monsoon during the last 25 ka. The general pattern and timing of monsoon variability in the Bay of Bengal and Andaman Sea seems to parallel the Arabian Sea, Africa, and Asian ice cores and speleothem records suggesting that a common tropical forcing may have induced these abrupt climate changes.