This study presents the first test of dysfunctional metacognitive beliefs as a predictor of anxiety symptom severity in youths following cognitive-behavioral therapy (CBT). N = 46 clinic-referred youths ages 6–16 years (M = 9.24, SD = 1.97) with anxiety disorders completed questionnaires on metacognitive beliefs and anxiety severity at CBT pretreatment and post-treatment. Overall metacognitive beliefs and all 5 subdimensions were significantly associated with anxiety severity at pretreatment. Metacognitive beliefs at pretreatment significantly predicted anxiety severity at post-treatment, and this predictive effect was moderated by age. In children under age 9 years, higher metacognitive beliefs predicted higher anxiety symptom severity at post-treatment. Changes in overall metacognitive beliefs, negative beliefs, and superstitious beliefs were significantly associated with changes in anxiety severity. Dysfunctional metacognitive beliefs, including heightened awareness of and focus on worries, portend a poorer CBT outcome in children with anxiety disorders. These findings support the promise of dysfunctional metacognitive beliefs for informing selection or tailoring of treatment.