Background Borderline personality disorder (BPD) and history of prior suicide attempt (SA) have been shown to be high predictors for subsequent suicide. However, no previous study has examined how both factors interact to modify clinical and suicide severity among adolescents. Methods This study presents a comprehensive assessment of 302 adolescents (265 girls, mean age = 14.7 years) hospitalized after a SA. To test clinical interactions between BPD and history of prior SA, the sample was divided into single attempters without BPD (non-BPD-SA, N = 80), single attempters with BPD (BPD-SA, N = 127) and multiple attempters with BPD (BPD-MA, N = 95). Results Univariate analyses revealed a severity gradient among the 3 groups with an additive effect of BPD on the clinical and suicide severity already conferred by a history of SA. This gradient encompassed categorical (anxiety and conduct disorders and non-suicidal-self-injury [NSSI]) and dimensional comorbidities (substance use and depression severity) and suicide characteristics (age at first SA). According to regression analyses, the BPD-MA group that was associated with the most severe clinical presentation also showed specific features: the first SA at a younger age and a higher prevalence of non-suicidal self-injury (NSSI) and anxiety disorders. The BPD-MA group was not associated with higher impulsivity or frequency of negative life events. Conclusions Based on these findings and to improve youth suicide prevention, future studies should systematically consider BPD and the efficacy of reinforcing early interventions for anxiety disorders and NSSI.