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This study evaluates the simulation of the seasonal cycle of water isotopic composition over Tibetan Plateau regions (TP) from six isotope‐enabled general circulation models (GCMs) participating in the second Phase of Stable Water Isotope Intercomparison Group. For both meteorological factors (precipitation rate and wind field) and isotopic composition, GCMs generally agree with reanalysis data and in‐situ observations, but there is a significant spread across models and the isotopic seasonality is systematically underestimated. In the southern TP, the precipitation isotopic composition is more depleted in summer than in winter, and the amplitude of the simulated isotopic seasonal variations is primarily driven by the amplitude of the simulated upstream precipitation. In contrast, in the northern TP, the precipitation isotopic composition is more depleted in winter than in summer, and the amplitude of the simulated seasonal variability of isotopes is mainly driven by the simulated strength of the zonal wind. We conclude that the skill of a GCM to simulate the seasonal cycle in the isotopic composition depends mainly on the skill of the GCM to simulate the Indian summer monsoon precipitation and the westerlies. The same causes contributing to the underestimated seasonality at present‐day may also contribute to the underestimated δ18O change at the mid‐Holocene. Plain Language Summary: The stable isotopic composition of water is determined by the relative abundances of heavier to lighter isotopologues. Isotope‐enabled general circulation models (GCMs) simulate water isotope composition considering several physical processes operating in the hydrologic cycle. Therefore, a comparison between observed and simulated values provides a better understanding of the processes constraining the isotope values. However, the simulated water isotopic composition is sensitive to various physical processes and parameterization schemes. In this study, we investigate how the model biases in the representation of atmospheric processes affect the simulation of isotopic composition over the Tibetan Plateau (TP). We compare the simulated isotopic composition in precipitation and water vapor to in‐situ and satellite measurements. Our analysis indicates that most GCMs underestimate the seasonality of the isotopic composition in the precipitation and vapor. In the southern TP, the spread of upstream precipitation is an important factor controlling the inter‐model spread in isotopic seasonality. In the northern TP, the westerlies majorly control isotope biases. Key Points: Models systematically underestimate the precipitation isotopic seasonality both in the southern and northern TPThe inter‐model spread of precipitation isotopic seasonality in the southern TP is driven by the spread of upstream precipitationThe westerlies dominate the inter‐model spread of precipitation isotopic seasonality in the northern TP [ABSTRACT FROM AUTHOR]