Adolescent brains are particularly susceptible to the rewarding properties of risky decisions in social contexts. Individual differences in genetic influences on dopamine transmission moderate neural outcome processing of risky decisions and may exert pronounced effects on adolescent risk-taking behavior (RTB) and corresponding neural outcome processing in peer contexts, a process called gene-environment interaction (G × E). Eighty-five undergraduate students completed a behavioral risk task alone and in the presence of a confederate peer providing “risky” feedback. We tested for G × E effects using a polygenic risk index that included 3 candidate genetic variations associated with high dopamine transmission efficiency, as well as the moderating role of family history of behavioral disinhibition. Difference waves for the P300 and FRN (i.e., feedback-related negativity) were examined as indices of neural outcome processing. A G × E effect was observed for RTB and the P300, but not the FRN. Family history of behavioral disinhibition also interacted with peer influence to predict P300 amplitude. These data provide preliminary evidence for G × E for peer-influenced RTB and neural outcome processing during late adolescence. Genetic influences on dopaminergic function may be particularly relevant for attentional and motivational neural systems, as indexed by the P300, which exert downstream effects on peer-influenced RTB.