Objective Although personality disorder not otherwise specified (PDNOS) is highly prevalent and associated with a high burden of disease, only a few treatment studies in this patient group exist. This study is the first to investigate the effectiveness of different modalities of psychotherapy in patients with PDNOS, i.e., short‐term (up to 6 months) and long‐term (more than 6 months) outpatient, day hospital, and inpatient psychotherapy. Method A total of 205 patients with PDNOS were assigned to one of six treatment modalities. Effectiveness was assessed over 60 months after baseline. The primary outcome measure was symptom severity, and the secondary outcome measures included psychosocial functioning and quality of life. The study design was quasi‐experimental, and the multiple propensity score was used to control for initial differences between treatment groups. Results All treatment modalities showed positive outcomes, especially in terms of improvements of symptom severity and social role functioning. At 12‐month follow‐up, after adjustment for initial differences between the treatment groups, short‐term outpatient psychotherapy and short‐term inpatient psychotherapy showed most improvement and generally outperformed the other modalities concerning symptom severity. At 60 months after baseline, effectiveness remained but observed differences between modalities mostly diminished. Conclusion Patients with PDNOS benefit from psychotherapy both at short‐term and long‐term follow‐up. Short‐term outpatient psychotherapy and short‐term inpatient psychotherapy seem to be superior to the other treatment modalities at 12‐month follow‐up. At 60‐month follow‐up, treatments showed mostly comparable effectiveness. Copyright © 2014 John Wiley & Sons, Ltd. Key Practitioner Messages The effectiveness of different modalities of psychotherapy in patients with PDNOS (i.e., short‐term vs long‐term; outpatient versus day hospital versus inpatient psychotherapy) has not yet been compared. Different modalities of psychotherapy are effective for patients with PDNOS, and positive effects remain after 5 years. In patients with PDNOS short‐term (less than 6 months) outpatient psychotherapy and short‐term inpatient psychotherapy seem to be superior to the four other treatment modalities at 12‐month follow‐up. At 60‐month follow‐up, treatments showed mostly comparable effectiveness.