Background Many psychological treatments have shown effect on reducing self‐harm in adults with borderline personality disorder. There is a need of brief psychotherapeutical treatment alternative for suicide prevention in specialized outpatient clinics. Methods/Design The DiaS trial was designed as a pragmatic single‐center, two‐armed, parallel‐group observer‐blinded, randomized clinical superiority trial. The participants had at least two criteria from the borderline personality disorder diagnosis and a recent suicide attempt (within a month). The participants were offered 16 weeks of dialectical behavior therapy (DBT) versus up to 16 weeks of collaborative assessment and management of suicidality (CAMS) treatment. The primary composite outcome was the number of participants with a new self‐harm (nonsuicidal self‐injury [NSSI] or suicide attempt) at week 28 from baseline. Other exploratory outcomes were: severity of borderline symptoms, depressive symptoms, hopelessness, suicide ideation, and self‐esteem. Results At 28 weeks, the number of participants with new self‐harm in the DBT group was 21 of 57 (36.8%) versus 12 of 51 (23.5%) in the CAMS treatment (OR: 1.90; 95% CI: 0.80–4.40; P = .14). When assessing the effect of DBT versus CAMS treatment on the individual components of the primary outcome, we observed no significant differences in the number of NSSI (OR: 1.60; 95% CI: 0.70–3.90; P = .31) or number of attempted suicides (OR: 2.24; 95% CI: 0.80–7.50; P = .12). Conclusion In adults with borderline personality traits and disorder and a recent suicide attempt, DBT does not seem superior compared with CAMS for reduction of number of self‐harm or suicide attempts. However, further randomized clinical trials may be needed.