It is important to provide treatment for juvenile and young adult violent offenders, since no or (too) late treatment increases the risk of recidivism and persistent aggressive behavior in adulthood. In this study, we investigated the efficacy of the intervention Responsive Aggression Regulation Therapy (Re-ART) Outpatient, which has been developed for adolescents and young adults between 16 and 24 years old with severe aggression problems. This quasi-experimental (pilot) study compared the effects of Re-ART Outpatient (n = 47) with a control group (n = 29) receiving treatment as usual on measures regarding risk of violent recidivism, impulsivity, emotional/personal functioning, family functioning, motivation, handling anger, self-reported problem behavior, coping skills, and cognitive distortions. Re-ART showed significantly better results than the control group on risk of violent recidivism, impulsivity, emotional/personal functioning, motivation, handling anger, certain coping skills, and certain cognitive distortions. Family functioning and self-reported rule-breaking behavior did not differ significantly between the groups. The results implicate that Re-ART is a promising outpatient intervention for young adults with severe aggressive behavior. The results can be explained by the fact that Re-ART treats both systemically and individually and pays specific attention to stress reduction and the improvement of executive functions.