Identity diffusion plays a central role in the onset of borderline personality disorders. Dialectical Behavioral Therapy for Adolescents (DBT-A) is a treatment program for adolescents with emotional instability and dysregulation. The interest of this study is to examine the potential effects of a standardized and certified DBT-A therapy program on the identity development of adolescents in an inpatient setting. A total of 138 adolescents aged 13 to 18 years with symptoms of severe emotional instability were assessed before and after a certified and standardized 12-week in-patient DBT-A program targeting emotional regulation with the following standardized instruments: the Assessment of Identity Development in Adolescence (AIDA), Scale of the Experience of Emotions (SEE), and Symptom Checklist (SCL-90-R). The results indicate a significant change in identity development, emotion regulation, and general symptoms of psychopathology after treatment with DBT-A. In this large sample of adolescents, DBT-A significantly improved identity development and reduced identity diffusion, however, without a treatment-as-usual control group as a limitation. Nevertheless, our results may become clinically relevant for the prevention of chronic impairment in emotionally unstable adolescents.