Biomass burning is a significant source of aerosol emissions in some regions, and it can have considerable impacts on regional climate. In particular, Earth system model simulations indicate that increased biomass burning aerosol emissions in the tropics through the 20th century contributed to statistically significant decreases in precipitation relative to the preindustrial era. In this study, we use the Community Earth System Model Large Ensemble (CESM1-LENS) experiment to evaluate the mechanisms by which biomass burning aerosols contributed to decreased tropical precipitation, focusing on South America and Southeast Asia. We utilize the all-but-one forcing simulations in which biomass burning aerosols are held constant while other forcings (e.g., greenhouse gases and land-use change) vary with time, which allows us to isolate the effect of biomass burning aerosol on precipitation and related processes. Our results demonstrate that multiple mechanisms contribute to decreased precipitation, including changes in cloud microphysics, the radiative effect of absorbing aerosols, and alterations in low-level regional circulation. Aerosol particles emitted by biomass burning increase the quantity of cloud condensation nuclei, which limits cloud droplet size and precipitation formation. Additionally, absorbing aerosols (e.g., black carbon) contribute to a warmer cloud layer, which promotes cloud evaporation, increases atmospheric stability, thus inhibiting convection, and alters regional circulation. These results bolster our understanding of processes that affect the water cycle in regions prone to biomass burning events, and they allow us to better anticipate future changes in precipitation that may occur as a result of changing aerosol concentrations.