Psychological studies have emphasized that motivation is regulated by the anticipation of the emotional impact from the possible occurrence of unexpected rewarding events. Here, we scrutinized the existence of a corresponding neural signal by means of event-related potentials (ERPs) and computational modeling. In the first experiment, we designed a task that manipulated the probability of gaining a monetary reward and measured ERPs during anticipation and at reward delivery. A sustained frontocentral neural activity (i.e., the stimulus preceding negativity, SPN) was evidenced during the anticipation period. Critically, the SPN was found to increase in amplitude as the reward became more unexpected. Changes in the SPN were found to be predictive of individual differences in risk seeking, suggesting that a greater risk attitude involved a greater motivational state for receiving an improbable reward. In the second experiment, SPN results associated with unexpected monetary gains were replicated in a condition in which participants avoided monetary losses and the occurrence of unexpected rewards was also associated with an increase in the amount of self-reported pleasure. These findings support the existence of a neural ERP signature that encodes the process of tuning our motivation to the possibility of receiving a desirable but improbable rewarding outcome.