This paper presents the results of studying the spatial distribution of the maximum monthly rainfall within the Kingdom of Saudi Arabia (KSA) using geographic information systems (GIS). Isohyetal maps were constructed based on long historical rainfall depth data (1963-2013) of 255 rain gauges, after processing the data using different interpolation methods and their conversion to grid raster. Six interpolation methods were used: (a) kriging, (b) spline, (c) natural neighbor, (d) inverse distance weighting (IDW), (e) modified Shepard, and (f) triangulation with linear interpolation (TIN). Estimations of the rainfall at the rain gauges were compared to the observed measurements as controls and the method that resulted in minimum residuals and minimum standard deviation; in this case, the IDW was selected for the study. The percentage of residuals within +/- 2.0 mm were found to be 98 % for IDW, modified Shepard, and TIN, 97 % for natural neighbor, 90 % for spline, and 32 % for kriging. While identifying potential water harvesting sites depends on many factors, such as watershed area, topography, morphology, and rainfall, the results of this study can be used for the primary selection of water harvesting sites based on rainfall being the most important factor.