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The impacts of future global warming on the climate of semi-arid to arid regions of Iran have remained mostly unexplored. Using Regional Climate Model version 4.4 (RegCM4) under the RCP4.5 and RCP8.5 scenarios, changes in surface water balance components in Iran are investigated for the future period 2070 to 2099 relative to the base period 1970 to 1999. The initial and boundary conditions for RegCM4 were derived from the CSIRO-Mk3.6 general circulation model (GCM). Model results during the base period are compared against the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) and the Climatic Research Unit (CRU) datasets. Analyses of the ERA-Interim and CRU datasets and model simulations during the base period indicated that due to the large latitudinal extent of Iran and its complex topography, different regions of the country experience a diverse range of climate, with monthly precipitation ranging from 0.1 mm day(-1) over arid regions of southeastern Iran to more than 2.5 mm day(-1) over wet coastal regions of northern Iran. In the future period, near-surface temperatures over all regions of Iran will increase under the two scenarios, with more temperature rises under the RCP8.5 scenario. Under the two scenarios, precipitation over most regions of Iran will increase in winter and decrease in summer. As winter will be warmer in the future under the two scenarios, and a warmer air has a higher water vapour storage capacity, precipitation will increase in the future winter climate. On the other hand, the decrease of precipitation in summer is due to an increase of the altitude of the lifting condensation level under the warmer climate. It is shown that increases in the surface temperature and precipitation in the future winter are associated with an increase of evaporatranspiration in nearly all regions of Iran.