Peers are thought to increase adolescents’ risk-taking behavior, at least in part, by heightening their sensitivity to rewards. In this study, we investigate whether the effect of peers on late adolescent males is exacerbated when youth are cognitively fatigued, a state characterized by weakened cognitive control and heightened orientation toward rewards, and well established as a factor that compromises decision making. We hypothesized that fatigued adolescents’ top-down regulation of reward-related impulses may be compromised, thereby potentially amplifying the effect of peers on reward- and risk-seeking behavior. Late adolescent males between 18 and 22 years old (mean age = 19.64, SD = 1.22; 61% Caucasian) completed a decision-making battery either alone or in the presence of 3 same-sex peers, and were either cognitively fatigued or non-fatigued. We compared behavior between four experimental groups—fatigued adolescents in a peer group, non-fatigued adolescents in a peer group, fatigued adolescents by themselves, and non-fatigued adolescents by themselves. The findings showed that cognitive fatigue and peer presence evinced independent effects on risk taking and sensitivity to rewards, but that these factors do not influence adolescent decision-making in an additive or synergistic fashion. To our surprise, being fatigued reduces (but does not eliminate) the effect of peers of risk taking. Moreover, the impact of peers on adolescent males’ ability to learn from negative consequences is not compromised when adolescents are in a state of mental fatigue. Our results suggest that mental fatigue increases late adolescent males’ reward sensitivity to the same extent as peer presence, but does not amplify the peer effect on risk-taking behavior. In this regard, grouping adolescents when they are fatigued may be less dangerous than when they are rested. Similarly, the added presence of peers does not further exacerbate the effect of fatigue on adolescent’s reward- and risk-seeking inclinations. In fact, given peers’ unique effect on adolescents’ ability to learn from costly decisions, our findings suggest that seeking the presence of peers—which is often a rewarding experience in and of itself—may be an adaptive response to mitigate the impact of fatigue on decision making.