Balancing instant gratification versus delayed but better gratification is important for optimizing survival and reproductive success. Although delayed gratification has been studied through human psychological and brain activity monitoring and animal research, little is known about its neural basis. We successfully trained mice to perform a waiting-for-water-reward delayed gratification task and used these animals in physiological recording and optical manipulation of neuronal activity during the task to explore its neural basis. Our results showed that the activity of dopaminergic (DAergic) neurons in the ventral tegmental area increases steadily during the waiting period. Optical activation or silencing of these neurons, respectively, extends or reduces the duration of waiting. To interpret these data, we developed a reinforcement learning model that reproduces our experimental observations. Steady increases in DAergic activity signal the value of waiting and support the hypothesis that delayed gratification involves real-time deliberation.