Objectives: Adolescents with substance use disorders (SUDs) exhibit high rates of comorbid psychological problems. This study aimed to examine the impact of an outpatient substance use treatment programme upon the psychological wellbeing of adolescents. Methods: A prospective study was carried out examining psychological symptoms in a group of adolescents attending the Youth Drug and Alcohol (YoDA) Addiction Service in Dublin. Participants were treated with evidenced based psychological models such as cognitive behavioural therapy, motivational interviewing and systemic family therapy. The Becks Youth Inventory was utilised to assess psychological symptoms at treatment entry and repeated three months later at follow up. Results: Among 36 adolescents who were included in this study, poly-substance misuse was the norm. Almost three-quarter had a cannabis use disorder (CUD). There were significant reductions in mean subscale scores of depression (56.0 to 50.8, p = 0.003), anger (55.2 to 49.5, p < 0.001) and disruptive behaviour (61.6 to 56.5, p = 0.002) at follow up. Although there wasn’t a statistically significant reduction in mean scores for anxiety, we observed a significant proportion of participants (p = 0.008) improving and moving out of a moderate to severe symptom range when examined by category. This was also the case for self-concept (p = 0.04). Furthermore this study revealed a positive correlation between the reduction in days of cannabis use and reduction in depressive scores (Pearson correlation 0.49, p = 0.01) among those with a CUD. Conclusion: The findings indicate that substance use treatment for adolescents is associated with important psychological and behavioural improvements.