The interpretation of variations in the global isotopic composition of precipitation and water vapor can be strengthened using an isotope-enabled atmospheric general circulation model (AGCM). Here we present a fast-physics atmospheric circulation model suitable for long ensemble integrations: the efficient AGCM SPEEDY, with newly added water isotope physics. The model (SPEEDY-IER) simulates the hydrological cycle and isotope ratios in atmospheric water at a fraction of the computational cost of IPCC-class GCMs. Despite its simplified physics, SPEEDY-IER captures many key features of the observed range of tropical, subtropical, and mid-latitude isotope variability when compared to the Global Network of Isotopes in Precipitation (GNIP), Stable Water Isotope Intercomparison-Group (SWING2) simulations, and satellite observations of isotopes in vapor. The incorporation of water isotopes in SPEEDY required two updates to the model's physics; namely, post condensational exchange associated with falling rain and soil hydrology. It is evident that these physical processes are essential for a skillful simulation of isotopes in precipitation and vapor. We conduct a suite of sensitivity tests to constrain effective parameters in the rain-exchange and land models, and assess the impact of the new physics to isotope simulations. The strong sensitivity to parameter choice in these components reaffirms the importance of land-atmosphere interactions and rain-vapor exchange on stable water isotope ratios in the atmosphere, and thus on the interpretation of paleoclimate records. The utility of SPEEDY-IER for climate applications is discussed.