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The Pamir range, located in Central Asia, mainly receives moisture from the mid-latitude westerlies, but its western side (i.e., Tajikistan Pamir) receives much of its precipitation in the winter and spring and its eastern side (i.e., Chinese Pamir) in the summer. Thus, the Pamir provides a natural laboratory to study the distribution of surface water stable isotopes across a large mountain range that ultimately receives moisture from one single source but has different precipitation seasonality regimes between its two sides. In this study, we present stable oxygen (δ18O) and hydrogen (δ2H) isotope data for 113 surface water samples from the Chinese Pamir. Our new data, along with previously published stable isotope data, show that the slope of the Chinese Pamir local meteoric water line is higher than that of the Global Meteoric Water Line (GMWL), and almost all of the data plot above the GMWL, implying that the Chinese Pamir surface waters have not experienced significant isotopic modification by evaporation. The Chinese Pamir surface waters have substantially higher δ18O and d-excess values and a steeper apparent δ18O lapse rate than surface water samples collected from the Tajikistan Pamir. We suggest that this contrast results from the shift in precipitation seasonality across the Pamir, with dominantly winter and springtime precipitation on the Tajikistan side and summertime precipitation on the Chinese side of the Pamir. This predominant summertime precipitation regime results in surface waters with high δ18O values in the Chinese Pamir. Further, this summertime moisture is dominantly convectively recycled moisture, resulting in high d-excess values in surface waters. The percentage of summertime moisture, which has high δ18O values, decreases west and with elevation in the Chinese Pamir, resulting in a steep apparent δ18O lapse rate of − 3.2 ‰/km. The importance of precipitation seasonality in modulating δ18O values across the Pamir suggests that proxy-derived records of past environments in the region must consider the mechanisms that today cause the seasonality contrast.