Non-suicidal self-injury has disseminated almost epidemically through the last decades in young persons. There is no evidence of any experimental treatment for non-suicidal self-injury in adolescents, superior to treatment as usual. Patients with stigmatizing illness such as non-suicidal self-injury may find internet-based interventions more acceptable. The study is a feasibility trial, with a randomization procedure in a parallel group design. Patients will be recruited from Child- and Adolescent Mental Health Services (age 13 to 17 years). The experimental intervention is internet-based, and therapist guided; ERITA consists of online Emotion Regulation Individual Therapy for Adolescents (ERITA) as add-on to treatment as usual. Treatment as usual is provided by multidisciplinary teams in nine outpatient clinics within Child and Adolescent Mental Health Services in the Capital Region of Denmark. Feasibility of recruitment and trial procedures will be monitored as well as explorative clinical outcomes.