Trauma-focused CBT (TF-CBT) is recommended by NICE for adolescents with post-traumatic stress disorder (PTSD). Whilst caregiver involvement is recommended, no specific guidance on the nature of involvement is provided although it is important that involvement takes account of the nature of the trauma. This case report details how Dummett’s systemic CBT formulation was used to plan an intervention such that systemic sessions were included in TF-CBT treatment of an adolescent female with PTSD which arose following interpersonal trauma. Trauma symptoms, trauma cognitions and depressive symptoms were measured throughout, with trauma symptoms showing clinically significant change over the course of therapy. Clinical implications are considered regarding the applicability of systemic CBT formulation and the inclusion of systemic sessions within TF-CBT for adolescents whose traumas arose interpersonally.