Narcissists are prone to risky decision-making, but why? This study tested—via behavioral and event-related potential (ERP) measures—two accounts: deficiencies in error monitoring and deficiencies in action updating. High and low narcissists were engaged in a monetary gambling task by choosing between a high-risk and a low-risk option while the electroencephalogram (EEG) was being recorded. Two ERP components relevant to outcome evaluation—feedback-related negativity (FRN) and P3—were analyzed, with the FRN serving as an index of error monitoring and the P3 as an index of action updating. Generally, high and low narcissists differed in the high-risk condition but not in the low-risk condition. At the behavioral level, high (vs low) narcissists made riskier decisions following high-risk decision outcomes, which was in line with past findings; at the neurophysiological level, while no FRN difference emerged between high and low narcissists, the outcome valence effect (positive vs negative) on the P3 was stronger among low narcissists than high narcissists following high-risk decision outcomes. One possible interpretation of the results is that narcissism is associated with reduced action updating. The findings contribute to the understanding of narcissistic decision-making and self-regulation.