In this pioneering study, we examined the influence of moral character attributes and responsible behavior on adolescent life satisfaction, as well as the mediating role of adolescents’ responsible behavior in the influence of moral character attributes on life satisfaction in 2,474 adolescents in Hong Kong. We used a 25-item measure to assess moral character attributes, a 15-item measure to assess responsible behavior, and the 5-item Satisfaction with Life Scale to assess life satisfaction. Factor analyses using confirmatory factor analyses provided support for the factorial validity of the measures of adolescent moral character attributes, responsible behavior, and life satisfaction. Consistent with our hypotheses, moral character attributes positively predicted adolescents’ responsible behavior as well as life satisfaction, and adolescents’ responsible behavior positively predicted life satisfaction. Separate analyses using PROCESS and combined analysis using SEM via Mplus showed that adolescents’ responsible behavior mediated the influence of moral character attributes on adolescent life satisfaction. The present findings highlight the importance of moral character on adolescent well-being and support the positive youth development approach in the field.