Although it is well established that transpiration contributes approximately half of all rainfall over Amazônia, it has been stubbornly unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite data sets to investigate the role of rainforest transpiration in the dry-to-wet season transition. We show that rainforest transpiration enables shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and leads to wet season onset. Further analysis suggests that aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results underscore the potential for deforestation to extend the dry season and enhance regional vulnerability to drought, and highlight the mechanisms by which interactions among land surface processes, atmospheric convection and aerosol pollution may alter the physics and timing of wet season onset. Examination of model outputs and recent reanalysis products in the context of these results reveals critical shortcomings in the representation of this transition.