Oxygen isotope (δ18O) records of two stalagmites from Tianmen Cave, on the south-central Tibetan Plateau, grew during much of Marine Isotope Stage 5. The Tianmen record, the first cave record from the Tibetan Plateau, characterizes a precipitation δ18O history larger in amplitude but similar in structure to Asian Monsoon records from the adjacent regions, providing essential evidence that the Asian Monsoon system, including the East Asian and Indian Monsoon subsystems, responds largely to changes in Northern Hemisphere summer insolation. Extremely low δ18O values during Marine Isotope Stages 5a, 5c, and 5e suggest that precipitation, rather than temperature, was a major factor controlling δ18O in precipitation on orbital time scales in the south-central Tibetan Plateau. The Tianmen record may help in the interpretation of regional ice core δ18O records. The large range of orbital-scale shifts in meteoric δ18O (>9‰) raises important considerations related to reconstructing the uplift history of the plateau.