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In a context of water scarcity in Morocco, the proper management of water resources is becoming a national priority. However, the strong spatiotemporal fluctuation of rainfall, considered as the first source of surface water intakes, complicates attempts to work on such a long-term purpose. In this work, we aim to provide an analysis of rainfall variability over the Oum Er-Rbia (OER) River basin, one of the most important basins in Morocco. For this, we assessed the annual rainfall deficit using the Standardized Precipitation Index (SPI) and then analyzed trends at annual, seasonal, and monthly bases using the Mann-Kendall test. The study was conducted on a set of data from 15 stations over a 40-years period (1970-2010). The results show that the OER River basin tends towards drier conditions. An abundance of deficit seasons has been noticed (50 to 63% of the seasons), especially after 1980-1981. Since then, the basin has known several deficit periods, sometimes of great spatial extent such as those that occurred in the 1980s and 1990s. Moreover, during the study period, the total annual rainfall has demonstrated a general decreasing tendency. The latter was mainly induced by changes in winter and spring months, which experienced some of the largest decline rates with a special abundant significance during the month of April.