Body dysmorphic disorder (BDD) is a common and impairing disorder that typically onsets before the age of 18. Although cognitive behavioral therapy (CBT) can effectively prevent severe long-term consequences of BDD if applied at an early age, it is scarcely available. To address this gap, Internet-delivered CBT (ICBT) was developed as a form of guided self-help, and was shown to be an effective treatment for adults with BDD. A pilot study by the authors of this project that assesses ICBT for adolescents with BDD is also showing promising treatment effects. This study will expand the pilot into a randomized controlled trial to test ICBT’s efficacy on 136 adolescents with BDD over the course of 14 weeks, compared to an Internet-based supportive therapy control. It will assess ICBT’s effects on BDD symptom severity, how well participants respond to treatment, and cost-effectiveness of the application. By examining these factors, this study hopes to show that evidence-based treatment for young people with BDD is effective – and available.