Previous studies suggested that trauma-focused cognitive behavior therapy (TF-CBT) is effective in reducing symptoms of post-traumatic stress disorder (PTSD) and depression in children and adolescents. However, the effects of app-assisted TF-CBT-based interventions in psychiatric clinics compared to treatment-as-usual (TAU) are under-explored. The purpose of this study was to reveal the added value of the app-assisted TF-CBT-based intervention as an add-on to TAU. The digitalized measures of the Child and Adolescent Trauma Screen (CATS), The Adolescent Dissociative Experience Scale (A-DES), Short Moods and Feelings Questionnaire (SMFQ), PTSD Checklist (PCL-5), and Children's Post-Traumatic Cognitions Inventory (CPTCI) were applied at baseline and post-treatment. The app-assisted TF-CBT-based group participants started with higher levels of negative alterations in cognition and mood compared to TAU participants but at post-treatment, no significant differences were observed between the app-assisted TF-CBT and TAU groups. Within the app-assisted TF-CBT group, post-treatment scores were significantly lower in post-traumatic stress symptoms, intrusion symptoms, alterations in arousal and reactivity, overall post-traumatic cognitions, and belief in being "a fragile person in a scary world”. Within the TAU group, post-treatment scores were notably lower just in dissociation and intrusion symptoms. The study suggests that app-assisted TF-CBT-based intervention can be effective in addressing specific trauma-related symptoms. The findings imply the necessity for screening for traumatic experiences, incorporating trauma-informed care, implementation of TF-CBT into treatment protocols, utilization of app-assisted interventions, tailoring interventions to individual needs, and comprehensive assessment and monitoring of treatment progress.