Metacognitive therapy (MCT) is a relatively new approach to treating mental disorders. The aim of the current meta-analysis was to examine the efficacy of MCT in patients with mental disorders. A comprehensive literature search revealed 16 published as well as unpublished studies on the efficacy of MCT, of which nine were controlled trials. These studies report on 384 participants suffering from anxiety or depression. Treatment efficacy was examined using a random effects model. On primary outcome measures the aggregate within-group pre- to posttreatment and pretreatment to follow-up effect sizes for MCT were large (Hedges' g = 2.00 and 1.65, respectively). Within-group pre- to posttreatment changes in metacognitions were also large (Hedges' g = 1.18) and maintained at follow-up (Hedges' g = 1.31). Across the controlled trials, MCT was significantly more effective than both waitlist control groups (between-group Hedges' g = 1.81) as well as cognitive behavior therapy (CBT; between-group Hedges' g = 0.97). Results suggest that MCT is effective in treating disorders of anxiety and depression and is superior compared to waitlist control groups and CBT, although the latter finding should be interpreted with caution. The implications of these findings are limited by small sample sizes and few active control conditions. Future studies should include larger sample sizes and also include comparisons of MCT with other empirically supported therapies. © 2014 Wiley Periodicals, Inc.