Global warming is expected to change surface water availability (precipitation minus evapotranspiration, P-E) and hence freshwater resources. However, the influence of land-atmosphere feedbacks on future P-E changes and the underlying mechanisms remain unclear. Here we demonstrate that soil moisture (SM) strongly impacts future P-E changes, especially in drylands, through regulating evapotranspiration and atmospheric moisture inflow. To do so, we use transient simulations from general circulation models, both with and without long-term SM changes, along with empirical statistical models of SM-atmosphere feedbacks. We find a consistent negative SM feedback on P-E, which may offset up to ~60% of the decline in dryland P-E that is otherwise expected to occur. The negative feedback is not caused by atmospheric thermodynamic responses, i.e., temperature and humidity changes, to declining SM, but rather by SM-related regulation of atmospheric circulation and vertical ascent that enhance moisture transport towards drylands. This SM effect is a large source of uncertainty in projected dryland P-E changes, underscoring the need to better constrain future SM changes and improve representation of SM-atmosphere processes in models.