Although adolescence is characterized by hormonal changes and increased disinhibited behaviors, explanations for these developmental changes that include personality and environmental factors have not been fully elucidated. We examined the interactions between psychosocial stress and the traits of negative emotionality and constraint on impulsive and risk-taking behaviors as well as salivary cortisol reactivity in 88 adolescents. In terms of behavioral outcomes, analyses revealed that negative emotionality and constraint were protective of impulsivity and risk taking, respectively, for adolescents in the no-stress condition; personality did not relate to either behavior in the stress condition. Low-constraint adolescents in the stress condition engaged in less risk taking than low-constraint adolescents in the no-stress condition, whereas there was no effect of stress group for high-constraint adolescents. In terms of cortisol reactivity, analyses revealed that low-constraint adolescents in the stress condition exhibited greater cortisol reactivity compared to high-constraint adolescents, which suggests that low-constraint adolescents mobilize greater resources (e.g., increased cognitive control, heightened attention to threat) in stressful situations relative to nonstressful ones. These results demonstrate that two facets of disinhibition and cortisol reactivity are differentially affected by psychosocial stress and personality (and their interactions) in adolescents.