Adolescence is characterized by increased risky decision-making, enhanced mesolimbic response to risk and reward, increased perceived stress, and heightened physiological response to stress relative to other age groups. In adults, evidence suggests that acute stress increases risky decision-making by stress-induced increases of dopamine in regions implicated in reward processing and decision-making. Acute stress also increases risky decision-making in adolescents, but the underlying neurobiological mechanisms remained unexplored. In this study, daily self-reports of stress were documented in adolescents and adults. Participants completed two fMRI visits during which they performed a risky decision-making task: once each when they endorsed a high and low level of stress. Results revealed that adolescent males took more advantageous risks under high stress relative to low stress whereas adult males took fewer non-advantageous risks under high stress relative to low stress. Adolescent males also showed a stress-related decrease in prefrontal activation when making risky decisions from high stress to low stress while adult males maintained prefrontal activation when making risky decisions across stress conditions. Adolescent and adult females did not exhibit stress-related changes in risky decisions. Moreover, greater prefrontal activation under stress was associated with fewer non-advantageous risks taken under stress. Implications for risk-taking under stress are discussed in light of these findings.