Objective: The study examined the relative efficacy of online (NET) versus clinic (CLIN) delivery of cognitive behavior therapy (CBT) in the treatment of anxiety disorders in adolescents. Method: Participants included 115 clinically anxious adolescents aged 12 to 18 years and their parent(s). Adolescents were randomly assigned to NET, CLIN, or wait list control (WLC) conditions. The treatment groups received equivalent CBT content. Clinical diagnostic interviews and questionnaire assessments were completed 12 weeks after baseline and at 6- and 12-month follow-ups. Results: Assessment at 12 weeks post-baseline showed significantly greater reductions in anxiety diagnoses and anxiety symptoms for both NET and CLIN conditions compared with the WLC. These improvements were maintained or further enhanced for both conditions, with minimal differences between them, at 6- and 12-month follow-ups. Seventy-eight percent of adolescents in the NET group (completer sample) no longer met criteria for the principal anxiety diagnosis at 12-month follow-up compared with 80.6% in the CLIN group. Ratings of treatment credibility from both parents and adolescents were high for NET and equivalent to CLIN. Satisfaction ratings by adolescents were equivalent for NET and CLIN conditions, whereas parents indicated slightly higher satisfaction ratings for the CLIN format. Conclusions: Online delivery of CBT, with minimal therapist support, is equally efficacious as clinic-based, face-to-face therapy in the treatment of anxiety disorders among adolescents. This approach offers a credible alternative to clinic-based therapy, with benefits of reduced therapist time and greater accessibility for families who have difficulty accessing clinic-based CBT. (Contains 5 tables and 1 figure.)