Social anxiety disorder (SAD) is a prevalent chronic condition with a large demand for treatment. This community outpatient study examined the effectiveness of a group intervention version of the established one-to-one cognitive therapy derived from the Clark and Wells model for SAD. Questionnaires were completed pre-treatment and post-treatment for SAD symptoms (Social Phobia Scale, Social Interaction Anxiety Scale), depressive symptoms (BDI-II), self-focused attention, safety behaviours (Social Phobia Weekly Summary Scale and Subtle Avoidance Frequency Examination), and impaired functioning (Work and Social Adjustment Scale). From an initial sample of 159 participants, 101 completed at least seven of the nine weekly group sessions (Mage = 34.1 years, SDage = 10.8 years, 53% female). Significant improvements were demonstrated on all measures. Large effect sizes were found for social anxiety symptoms and safety behaviour use. Self-focused attention, depressive symptoms, and impaired functioning had moderate effect sizes. Effect sizes for anxiety (d = 1.00 and 1.32) and mood measures (d = 0.71) were as high, or in some cases, higher than previous group treatment studies. Results suggest group cognitive therapy for SAD based on the Clark and Wells model is effective in a clinical setting for individuals with moderate/severe and treatment-resistant social anxiety.