Multidisciplinary treatment programs for patients with personality disorders (PDs) often include art therapy, but the efficacy of this intervention has hardly been evaluated. The objective of this study is to evaluate the effects of an art therapy intervention on psychological functioning of patients with a PD. In this randomized controlled trial, 57 adult participants diagnosed with a PD cluster B/C (SCID-II) were randomly assigned to either weekly group art therapy (1.5 hours, 10 weeks) or a waiting list group. Outcome measures OQ45, AAQ-II, and SMI were assessed at baseline, at post-test (10 weeks after baseline), and at follow-up (5 weeks after post-test). The results show that art therapy is an effective treatment for PD patients because it not only reduces PD pathology and maladaptive modes but it also helps patients to develop adaptive, positive modes that indicate better mental health and self-regulation.