During the COVID-19 epidemic, face-to-face mental health services faced obstacles. Using Internet-based interventions was a good solution and had the potential to overcome these treatment barriers. However, there is no strong research evidence about the effectiveness of these methods for social anxiety disorder in different cultures and developing countries. Therefore, the present study aimed to investigate the effectiveness and application of Internet-based cognitive-behavioral therapy for social anxiety disorder in Iran. The current study was a pretest-posttest follow-up experimental design. Fifty-four adolescents with social anxiety disorder were selected from Lorestan province (Iran) by cluster sampling method and randomly assigned to three groups: face-to-face, internet-based, and wait-list control. At the beginning and end of the study and 3-month follow-up, three groups were interviewed and answered questionnaires related to the primary and secondary symptoms of social anxiety disorder. Two experimental groups were treated with the same therapeutic intervention during 10 weekly sessions. ANCOVA analysis showed that both forms of intervention effectively reduced social phobia, fear of negative evaluation and social interaction anxiety and increased emotion regulation. Also, a significant decrease in secondary outcomes, including physical symptoms, insomnia, social dysfunction, and depression symptoms, was observed in both groups. The treatment effects were stable during a 3-month follow-up. Our findings showed that although Internet-based cognitive-behavioral therapy for adolescents with social anxiety disorder can be effective, several clinical, cultural, and implementation weaknesses should be considered.