This study examined how non-suicidal self-injury disorder (NSSID) differs with respect to mental disorders, gender and attachment status in adolescent psychiatric patients. In particular, we analyzed attachment-related traumatic material underlying adolescent NSSID. Our sample consisted of 137 in-patient adolescents aged 12 to 18 years (73% female, M = 15.09, SD = 1.44; 27% male, M = 14.65, SD = 1.53). Forty-four patients (32.1%) fulfilled the diagnostic criteria for NSSID according to the DSM-5 and ninety-three patients (67.9%) did not meet diagnostic criteria for NSSID. Our results revealed a higher prevalence of NSSID in female patients and in patients with mood disorders. In the total sample, 52% of our in-patients were classified with an unresolved attachment status. The diagnostic subgroup analysis demonstrated a higher percentage of unresolved attachment status only in patients with eating disorders and NSSID. However, our in-depth analysis of the total sample revealed that patients with NSSID demonstrated more traumatic material in their attachment interviews indicating a greater severity of attachment trauma. In particular the theme of helplessness in interpersonal conflicts left them in a state of attachment dysregulation. Intervention strategies targeting traumatic attachment-related themes might be useful to reduce the number of adolescents engaging in NSSI.