Accurate forecast of the Potential Evapotranspiration (PET) at a location (where station observation is not available) is necessary in arid/hyper-arid regions (e.g., Egypt) to monitor daily agricultural activities. The Penman-Monteith equation is the standard physical method to compute the PET, but it requires many variables (mostly are calculated empirically). Instead, the Hargreaves-Samani (HS) method was used because it is recommended by the Food and Agriculture Organization and it requires only two variables: global incident solar radiation and daily mean air temperature. Additionally, regional climate models (e.g., RegCM4) can be an alternative tool to estimate the PET constrained by a long-term gridded PET data (Climate Research Unit; CRU) at any location. To accomplish this task, a 39-year simulation was conducted. The RegCM4 was driven by the ERA-Interim reanalysis with 60 km grid spacing. Preliminary results indicated that the RegCM4 was able to capture the monthly variability of the simulated PET with respect to the CRU; however the model overestimates the PET particularly in the summer months (June, July and August). Over all considered locations, performance of the RegCM4 was notably improved when a linear regression model (LRM; between RegCM4 and CRU) was used (indicated by a low bias between the corrected RegCM4 and CRU). In conclusion, the RegCM4 model can accurately calculate the PET at the location of interest by means of the HS equation and a LRM either in the present climate or under different future scenarios.