Objective: To examine whether changes in cognitive reappraisal self-efficacy (CR-SE) mediate the effects of individually administered cognitive-behavioral therapy (I-CBT) for social anxiety disorder (SAD) on severity of social anxiety symptoms. Method: A randomized controlled trial in which 75 adult patients (21-55 years of age; 53% male; 57% Caucasian) with a principal diagnosis of generalized SAD were randomly assigned to 16 sessions of I-CBT (n = 38) or a wait-list control (WL) group (n = 37). All patients completed self-report inventories measuring CR-SE and social anxiety symptoms at baseline and post-I-CBT/post-WL, and I-CBT completers were also assessed at 1-year posttreatment. Results: Compared with WL, I-CBT resulted in greater increases in CR-SE and greater decreases in social anxiety. Increases in CR-SE during I-CBT mediated the effect of I-CBT on social anxiety. Gains achieved by patients receiving I-CBT were maintained 1-year posttreatment, and I-CBT-related increases in CR-SE were also associated with reduction in social anxiety at the 1-year follow-up. Conclusions: Increasing CR-SE may be an important mechanism by which I-CBT for SAD produces both immediate and long-term reductions in social anxiety. (Contains 3 tables, 2 figures and 1 footnote.)