Background <p>Acceptance and Commitment Therapy (ACT) has shown promising results in addressing antisocial and criminal behaviors. This quasiexperimental study with a control group and repeated measures design was conducted to investigate the effects of the ACT-based intervention on psychological flexibility, empathy, and anger in adolescents involved in crime.</p> Methods <p>The study sample consisted of 24 adolescents who had previously been involved in a forensic investigation. Among them, 12 were assigned to the intervention group, and 12 were assigned to the control group. While the participants in the intervention group underwent the ACT-based intervention one session per week for six weeks, the participants in the control group underwent no intervention. Measurements were conducted at four different times: before the intervention, after the intervention, at the one-month follow-up and at the three-month follow-up.</p> Results <p>The findings indicated that the intervention based on the ACT significantly increased psychological flexibility in adolescents involved in crime, and these improvements were sustained at both follow-up assessments. The empathy scores, including emotional and cognitive empathy scores, gradually increased over time in the intervention group, whereas they remained stable or slightly decreased in the control group. With respect to anger, the adolescents in the intervention group presented decreases in trait anger and anger-out expression, along with increases in anger control. No improvements were observed in the control group, and in some cases, the outcomes worsened over time. The positive changes achieved through the intervention were largely maintained during the follow-up period.</p> Conclusions <p>These results suggest that ACT may be an effective therapeutic approach for addressing behavioral and emotional difficulties in this disadvantaged population.</p> Trial registration <p>ISRCTN Registry, ISRCTN11760715. Registered on 20 October 2025. Retrospectively registered.</p>