Numerous randomized controlled trials have shown cognitive behaviour therapy (CBT) to be effective in treating social anxiety disorder (SAD). Yet, less is known about the effectiveness of CBT for SAD conducted by psychotherapists in training in routine clinical practice. In this study, 231 patients with SAD were treated with CBT under routine conditions and were examined at pre- and post-treatment as well as at 6 and 12 months follow-up. We applied self-reports to assess symptoms of SAD (defined as primary outcome), depression and psychological distress (defined as secondary outcome). We conducted both completer and intent-to-treat analyses and also assessed the reliability of change with the reliable change index. Results revealed significant reductions in symptoms of SAD between pre- and post-assessments, with effect sizes ranging from d = 0.9 to 1.2. Depending on the SAD specific questionnaire applied, 47.8% to 73.5% of the sample showed a reliable positive change, whereas 1.9% to 3.8% showed a reliable negative change. Depressive symptoms and psychological distress also decreased significantly from pre- to post-assessment, with large effect sizes. Significant treatment gains regarding both primary and secondary outcomes were further observed at 6 and 12 months follow-up. The current findings based on a large sample of patients suggest that psychotherapists in CBT training working under routine conditions can effectively treat symptoms of SAD, depression and psychological distress.