Justice-involved adolescents engage in high levels of risky sexual behavior, underscoring the need for targeted, effective, prevention interventions geared toward this population. In a randomized controlled trial, 484 detained adolescents received a theory-based intervention or an information-only control. We have previously demonstrated that the theory-based intervention was superior to the control condition in changing theoretical mediators and in producing longitudinal decreases in risky sexual behavior. In the present study, we examined differential response to the intervention based on the adolescents' level of positive outlook (composed of self-esteem, perceived control over the future and optimism toward the future). Changes to putative theoretical mediators (attitudes, perceived norms, self-efficacy and intentions) were measured immediately post-intervention, and behavioral data were obtained 3, 6, 9 and 12 months later. Positive outlook significantly moderated program effects both in the context of the mediational path model and in the context of the longitudinal growth model. Specifically, intervention effects were strongest for those scoring relatively lower on the positive outlook dimension, whereas adolescents high in positive outlook demonstrated greater attitudes and self-efficacy and decreased risky sexual behavior, regardless of condition. Findings are discussed in terms of targeting and tailoring of intervention content.