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West Africa faces an increasing challenge related to water resources in the coming decades; hence high-confidence climate projection is crucial for the region's future. In this study, future changes in summer monsoon rainfall under the RCP8.5 scenario were investigated over West Africa for the period of 2074-2099 with respect to the present-day period of 1980-2005 using the Rossby Centre Regional Climate Model (RCA4), driven by six Coupled Model Intercomparison Project phase 5 (CMIP5) global circulation models (GCMs). Through evaluation of the present climate, the RCA4 simulations and their multi-model ensemble mean (EnsMean) reasonably reproduced the climatology of the summer rainfall and associated tropospheric circulation features. In a warmer climate, summer monsoon rainfall is projected to increase in most parts of West Africa, but a decrease is expected in the northwest, around Senegal. Changes in the monsoon circulation are opposite in the lower and upper troposphere, with the tipping point at 400hPa. Specifically, a projected strengthening and northward shift is observed at the lower and mid-tropospheric level, while weakening is evident at the upper tropospheric level. A diagnosis based on a moisture budget equation reveals that the robust positive response of West African summer monsoon rainfall to global warming is largely explained by the enhancement of moisture convergence and surface evaporation.