Background: Post-traumatic stress disorder (PTSD) with co-occurring severe psychopathology such as borderline personality disorder (BPD) is a frequent sequel of childhood sexual abuse (CSA). CSA-related PTSD has been effectively treated through cognitive-behavioural treatments, but it remains unclear whether success can be achieved in patients with co-occurring BPD. The aim of the present study was to determine the efficacy of a newly developed modular treatment programme (DBT-PTSD) that combines principles of dialectical behaviour therapy (DBT) and trauma-focused interventions. Methods: Female patients (n = 74) with CSA-related PTSD were randomised to either a 12-week residential DBT-PTSD programme or a treatment-as-usual wait list. About half of the participants met the criteria for co-occurring BPD. Individuals with ongoing self-harm were not excluded. The primary outcomes were reduction of PTSD symptoms as assessed by the Clinician-Administered PTSD Scale (CAPS) and by the Posttraumatic Stress Diagnostic Scale (PDS). Hierarchical linear models were used to compare improvements across treatment groups. Assessments were carried out by blinded raters at admission, at end of treatment, and at 6 and 12 weeks post-treatment. Results: Under DBT-PTSD the mean change was significantly greater than in the control group on both the CAPS (33.16 vs. 2.08) and the PDS (0.70 vs. 0.14). Between-group effect sizes were large and highly significant. Neither a diagnosis of BPD nor the severity or the number of BPD symptoms was significantly related to treatment outcome. Safety analyses indicated no increase in dysfunctional behaviours during the trial. Conclusion: DBT-PTSD is an efficacious treatment of CSA-related PTSD, even in the presence of severe co-occurring psychopathology such as BPD.