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Stratospheric Aerosol Geoengineering (SAG) is proposed to offset global warming; however, the use of this approach can an impact on the hydrological cycle. We used simulations from Coupled Model Intercomparison Project (CMIP5) and Geoengineering Model Intercomparison Project (G3 simulation) to analyze the impacts of SAG on precipitation (P) and to determine its responsible causes in West Africa and Sahel region. CMIP5 historical data were first validated, the results obtained are consistent with observational data. Under the Representative Concentration Pathway scenario RCP4.5, a slight increase is found in the West Africa Region relative to present‐day climate. The dynamic processes especially, the monsoon shifts are responsible for this precipitation change. Under RCP4.5, during the monsoon period, reductions in P are 0.86%, 0.80% relative to the present‐day climate in the Northern and Southern Sahel, respectively, while precipitation is increased by 1.04% in the West African Region. Under SAG, we find a 3.71% decrease of precipitation in the West African Region while the precipitation decrease is 17.4% and 8.47% respectively in the North Sahel and South Sahel. This decrease in monsoon precipitation is mainly explained by changes in dynamics, which lead to weakened monsoon circulation and a shift in the distribution of monsoon precipitation. This result suggests that SAG deployment to balance all warming can be harmful to rainfall in WAR if the amount of SO2 to be injected into this tropical area is not taken into consideration. Plain Language Summary: Stratospheric Aerosol Geoengineering (SAG) deployment has been proposed to reduce temperature increase in the context of global warming but its impacts on the hydrological cycle and the underlying causes need to be evaluated on the regional scale. While some effects of Stratospheric Aerosol Geoengineering on the water cycle are uncertain, various studies suggest that there could be considerable changes in regional rainfall. Climate simulations (Coupled Model Intercomparison Project: CMIP5 and Geoengineering Model Intercomparison Project: GeoMIP) are used in this work to quantify the impacts on the monsoon rainfall in West Africa. We determine the changes in precipitation and the mechanisms responsible for these changes in West Africa during summer using Stratospheric Aerosol Geoengineering. Under global warming, while a slight decrease of precipitation is noted in the Sahel region, a significant decrease in rainfall is obtained over the West Africa region and the Sahel region under SAG. The main processes responsible for the changes of precipitation under SAG are determined based on the decomposition approach. Results show that changes in precipitation are largely related to changes in dynamic processes (monsoon circulation). Key Points: We determine the changes of West African Summer Monsoon precipitation using stratospheric aerosol geoengineering injections climate modelsIncrease of precipitation is presented under global warming while its decrease is obtained with stratospheric aerosol geoengineering modelsThese changes of West African Summer Monsoon precipitation are mainly driven by the dynamic processes [ABSTRACT FROM AUTHOR]