We examined the role of hope in the behavioral and socioemotional outcomes of adolescents referred to psychotherapy. Adolescents (n = 3517), aged 12-17 years, and their parents rated adolescents' behavior problems, adolescents' adaptive behavior, and their own subjective experience of hopefulness at intake and at 3-month follow-up. Therapists also rated adolescents' behavior problems and adaptive behavior at intake and follow-up. Adolescents who reported increased hopefulness during treatment showed significantly fewer problems and significantly greater adaptive behavior at follow-up than adolescents who reported stable or decreased hopefulness. Similarly, adolescents' outcomes were significantly associated with parents' change in hopefulness during treatment. Results were consistent for adolescent, parent, and therapist ratings of outcomes. Our findings provide some of the first empirical evidence for the importance of hopefulness in the psychosocial treatment of adolescents and highlight the need to attend to hopefulness in both youths and their parents.