Background: Borderline personality disorder (BPD) is known to be associated with high rates of comorbidity and severe impairment of psychosocial functioning in adults. The aim of this study was to investigate Axis I and Axis II disorders, as well as psychosocial functioning, in a clinical sample of adolescents with BPD and to compare these with participants with mixed psychiatric diagnoses. Methods: Female adolescent patients were consecutively recruited from the child and adolescent psychiatry department of a university hospital. Axis I and Axis II diagnoses were assessed by experienced clinicians using well-established semistructured interviews, along with psychosocial functioning. Results: The final sample (87 participants) comprised 31 participants with a diagnosis of BPD and 56 participants with mixed psychiatric diagnoses. The most common comorbid disorders in the adolescent BPD sample were mood, eating, dissociative, and substance use disorders in Axis I, and cluster C personality disorders in Axis II. The BPD group showed a significantly higher average number of comorbid Axis I and Axis II diagnoses and significantly lower psychosocial functioning compared with the clinical control group. Regression analyses revealed that psychosocial functioning was predicted by socioeconomic status and comorbid disorders, as well as the unique influence of BPD itself. Conclusion: Adolescent BPD in females is accompanied by high rates of psychiatric comorbidity and poor psychosocial functioning. This underscores the need for diagnosis of BPD at its early stages, in order to facilitate appropriate interventions.