So far, the specific role of the ventral striatum in reward-based decision making remains elusive. Here, we examined the role of the ventral striatum in reward-based decision making using functional MRI and a probabilistic object reversal task. During decision making, activity in the ventral striatum increased monotonically as a function of association learning and was greatest when the individuals expected to be rewarded for the decision with high certainty. Conversely, during the reward phase, activity in the ventral striatum showed an inverted U-shaped modulation by learning and was greatest when uncertainty about the outcome was maximal. Our data indicate that, during reward-based decision making, the ventral striatal signal dynamically changes over time dependent on the phase of the reward process and on the learning status and thereby acts as a motivational engine for the continuation of behavior.