Background The current study evaluated the effectiveness and safety of intensive prolonged exposure (PE) targeting adolescent patients with complex posttraumatic stress disorder (PTSD) and comorbid disorders following multiple interpersonal trauma. Methods Ten adolescents meeting full diagnostic criteria for PTSD were recruited from a specialized outpatient mental health clinic and offered a standardized intensive PE. The intensive PE consisted of three daily 90‐min exposure sessions delivered on five consecutive weekdays, followed by 3 weekly 90‐min booster sessions. In a single‐trial design, the participants were randomly allocated to one of five baseline lengths (4–8 weeks) before starting the intensive PE. Before, during, and after intensive PE completion, self‐reported PTSD symptom severity was assessed weekly as a primary outcome (a total of 21 measurements). Furthermore, clinician‐administered PTSD diagnostic status and symptom severity (primary outcome), as well as self‐reported comorbid symptoms (secondary outcomes), were assessed at four single time points (baseline‐to‐6‐month follow‐up). Results Time‐series analyses showed that self‐reported PTSD symptom severity significantly declined following treatment (p = .002). Pre‐postgroup analyses demonstrated significant reductions of clinician‐administered PTSD symptom severity and self‐reported comorbidity that persisted during the 3‐ and 6‐month follow‐ups (all ps < .05), where 80% of adolescents had reached diagnostic remission of PTSD. There was neither treatment dropout nor any adverse events. Conclusions The results of this first proof of concept trial suggest that intensive PE can be effective and safe in an adolescent population with complex PTSD, although the gains achieved need to be confirmed in a randomized controlled trial.