Risk behavior during adolescence results in substantial morbidity and mortality. Sensation seeking consistently relates to engagement in risk behavior, but the psychological mediators of this relationship remain unclear. The current study demonstrates that adolescents' judgments of the costs versus benefits of risk behavior were a significant mediator of this relationship. Participants were 406 racially and ethnically diverse adolescents ages 12-17 (M = 14.5, SD = 1.7; 48.3% female) who participated in a larger multi-site investigation of personality and neurocognitive predictors of risk behavior. Data were collected via self-report in a single laboratory session. Mediation of the relationship of sensation seeking to risk behavior was tested using structural equation modeling. Results indicated that higher levels of sensation seeking predicted weighing the benefits of risk behavior higher than its costs, which in turn predicted higher levels of risk behavior. Implications of these findings for understanding mechanisms underlying adolescents' risk behavior and directions for future research are discussed.