Background <p>To evaluate the effectiveness of mentalization-based treatment (MBT) for borderline personality disorder (BPD) compared to structured clinical management (SCM) in a South Korean clinical setting.</p> Methods <p>This single-blind pilot randomized controlled trial included 64 individuals with a primary diagnosis of BPD who underwent either MBT (<i>n</i> = 37) or SCM (<i>n</i> = 27) in an outpatient setting over 12 months. Both interventions included weekly individual and group sessions. The primary outcome measure was the number of suicide attempts. Secondary outcomes included depressive symptoms, suicide risk, hospitalization frequency and duration, mentalization difficulties, adult attachment styles, BPD symptom severity, and resilience. Outcomes were assessed at baseline and 6 and 12 months. Analyses were conducted using generalized estimating equations, following the intention-to-treat principle.</p> Results <p>Both treatment groups demonstrated a significant reduction in suicide attempts over 12 months, with no significant between-group differences in the primary outcome. However, participants receiving MBT showed consistent and sustained improvements in depressive symptoms, mentalization difficulties, and resilience. MBT was also associated with a significant reduction in attachment anxiety at 12 months, whereas the SCM produced earlier reductions in suicide risk and improvements in psychic equivalence. No significant changes were observed in attachment avoidance or in the hasty and incomplete mentalizing domains in either group.</p> Conclusions <p>This pilot trial provides preliminary evidence that both MBT and SCM were effective in reducing suicidal behaviors and enhancing psychological functioning among Koreans with BPD. MBT demonstrated greater long-term benefits in emotion regulation and adaptability, suggesting that it may serve as a culturally scalable intervention in non-Western clinical settings.</p>