OBJECTIVES: To evaluate the cost-effectiveness of a therapist-guided Internet-delivered cognitive behavioral therapy (ICBT) intervention for adolescents with obsessive-compulsive disorder (OCD), compared to untreated patients on a waitlist. METHODS: Sixty-seven adolescents (12–17yrs) with a DSM-5 diagnosis of OCD participated in a 12-week, single-blinded randomized controlled trial comparing ICBT to a waitlist control. Cost data were collected at baseline and after treatment, including health care use, supportive resources, prescription drugs, prescription-free drugs, school absence and productivity loss, as well as the cost of ICBT. Health outcomes were defined as treatment responder rate and Quality Adjusted Life Years (QALYs) gain. Bootstrapped mixed model analyses were conducted comparing incremental costs and health outcomes between the groups, from the societal and healthcare perspectives. RESULTS: Compared to waitlist control, ICBT generated substantial societal cost savings averaging -150.63USD (95% CI [-166.02, -135.23]) per patient. The cost reductions were mainly driven by reduced health care use in the ICBT group. From the societal perspective, the probability of ICBT being cost-saving compared to waitlist control was approximately 60%. From the healthcare perspective, the cost for an additional percentage increase in responder rate was approximately 80 USD. CONCLUSIONS: The results suggest that therapist-guided ICBT is a cost-effective treatment and results in societal cost savings, compared to leaving patients untreated. Since, at present, most patients with OCD do not have access to evidence-based treatments, the results have important implications for the increasingly strained national and healthcare budgets. Future studies should compare the cost-effectiveness of ICBT with regular face-to-face CBT.