Outcome uncertainty plays an important role in decision making. The present study used event-related potentials (ERP) combined with a simple gambling task to investigate the time course of outcome uncertainty processing in the gain and the loss domains. Behaviorally, participants showed a preference for the uncertain option in the gain domain, whereas they showed no such preference in the loss domain. In ERP analyses, the feedback-related negativity (FRN) amplitudes mirrored the behavioral results: for gains, cue-elicited FRN amplitudes were more negative for uncertain than for certain cues, whereas for losses, cue-elicited FRN amplitudes for certain cues and uncertain cues were not different. In contrast, the P300 amplitude was sensitive to the valence of cues (gains or losses) but not to the uncertainty of the outcome. These results suggest that the outcome uncertainty is rapidly and differentially processed in the gain and loss domains.