Introduction: Borderline personality disorder (BPD) is a severe and prevalent psychiatric disorder. Mentalization-based therapy (MBT) is an evidence-based intervention for BPD, and several countries offer treatment programs for BPD lasting for years, which is resource demanding. No previous trial has compared short-term with long-term MBT. Objective: The aim of the study was to assess the efficacy and safety of short-term versus long-term MBT for outpatients with BPD. Methods: Adult outpatients (≥18 years) with subthreshold or diagnosed BPD were randomly assigned (1:1) to short-term MBT (5 months) or long-term MBT (14 months). The primary outcome was BPD symptoms assessed with the Zanarini Rating Scale for Borderline Personality Disorder. Secondary outcomes were functional impairment, quality of life, global functioning, and severe self-harm. All outcomes were primarily assessed at 16 months after randomization. Results: Between October 4, 2018, and December 3, 2020, we randomly assigned 166 participants to short-term MBT (n = 84) or long-term MBT (n = 82). Regression analyses showed no evidence of a difference when assessing BPD symptoms (MD 0.99; 95% CI: -1.06 to 3.03; p = 0.341), level of functioning (MD 1.44; 95% CI: -1.43 to 4.32; p = 0.321), quality of life (MD -0.91; 95% CI: -4.62 to 2.79; p = 0.626), global functioning (MD -2.25; 95% CI: -6.70 to 2.20; p = 0.318), or severe self-harm (RR 1.37; 95% CI: 0.70-2.84; p = 0.335). More participants in the long-term MBT group had a serious adverse event compared with short-term MBT (RR 1.63; 95% CI: 0.94-3.07; p = 0.088), primarily driven by a difference in psychiatric hospitalizations (RR 2.03; 95% CI: 0.99-5.09; p = 0.056). Conclusion: Long-term MBT did not lead to lower levels of BPD symptoms, nor did it influence any of the secondary outcomes compared with short-term MBT.