Chinese speleothems (cave deposits) preserve a remarkable paleoclimate record in their oxygen isotope ratios (δ18O); the precise interpretation of this record has been the subject of stimulating discussion. Most studies link the δ18O variability in Chinese speleothems to regional summer monsoon rainfall and/or rainfall integrated between tropical sources and cave sites. Discussion has centered on mechanisms behind this link as well as the location and seasonality of hypothesized rainfall changes. Until now, these hypotheses were not directly tested in speleothems because conventional drill sampling techniques are insufficient for measuring speleothem δ18O at seasonal resolution. Here we use an ion microprobe to analyze seasonal δ18O variability in an annually banded stalagmite from Kulishu Cave (northeastern China) that grew during the last deglaciation. The new seasonal resolution data show that the stalagmite δ18O values record two aspects of regional monsoon dynamics: (1) changes in the isotopic fractionation of water vapor sourced from both the Indian and Pacific Oceans, and (2) the annual proportion of summer monsoon rainfall, which was systematically greater during the Holocene and Bølling-Allerød than during the Younger Dryas. Both relate to regional rainfall; the isotopic fractionation changes also relate to rainfall integrated from tropical sources.