In order to reconstruct the activity of the Indian Summer Monsoon (ISM) during the last ∼4 kyr, two stalagmites from Baratang cave in Andaman Islands have been investigated for their temporal variations in δ18O. The chronology is provided by radiocarbon ages on the stalagmites. During 1800–2100 cal yr BP, we observed a significant increase in stalagmite δ18O that we infer is caused by the amount effect. This increase implies a strong reduction in the strength of the ISM that is correlated with the Roman Warm Period. Other proxy records in the region confirm a reduction in ISM activity during this time. Reduction in the ISM is also observed around 1500 and 400–800 cal yr BP; the latter period is the transition from the Medieval Warm Period to the Little Ice Age. The strongest monsoon in the last ∼4 kyr is observed during 800–1200 cal yr BP, the Medieval Warm Period.