The central-eastern Sahel (CES) subregion of West Africa has experienced a significant increasing trend in summer monsoon rainfall during 1980–2012, but the reasons remain unclear. In this study, we employ a moisture budget analysis to assess the controlling mechanisms and to quantify the contributions of different factors to this increasing trend. Results reveal that the majority of the increasing rainfall over the CES is balanced by increased vertical moisture advection. We further decompose the vertical moisture advection term into dynamic, thermodynamic, and nonlinear components. The dynamic component exhibits the largest contribution to the trend, followed by the thermodynamic component. These components are modulated by strong convergence in the middle and lower troposphere and increased near-surface specific humidity from a remote source. Diagnosing changes in vertical moisture advection against the background of global warming deserves more attention when projecting changes in climate at subregional scales, especially in arid and semiarid regions.