Importance: Cognitive behavioral therapy is recommended for anxiety-related disorders, but evidence for its long-term outcome is limited. Objective: This systematic review and meta-analysis aimed to assess the long-term outcomes after cognitive behavioral therapy (compared with care as usual, relaxation, psychoeducation, pill placebo, supportive therapy, or waiting list) for anxiety disorders, posttraumatic stress disorder (PTSD), and obsessive-compulsive disorder (OCD). Data Sources: English-language publications were identified from PubMed, PsycINFO, Embase, Cochrane, OpenGrey (1980 to January 2019), and recent reviews. The search strategy included a combination of terms associated with anxiety disorders (eg, panic or phobi) and study design (eg, clinical trial or randomized controlled trial). Study Selection: Randomized clinical trials on posttreatment and at least 1-month follow-up effects of cognitive behavioral therapy compared with control conditions among adults with generalized anxiety disorder, panic disorder with or without agoraphobia, social anxiety disorder, specific phobia, PTSD, or OCD. Data Extraction and Synthesis: Researchers independently screened records, extracted statistics, and assessed study quality. Data were pooled using a random-effects model. Main Outcomes and Measures: Hedges g was calculated for anxiety symptoms immediately after treatment and at 1 to 6 months, 6 to 12 months, and more than 12 months after treatment completion. Results: Of 69 randomized clinical trials (4118 outpatients) that were mainly of low quality, cognitive behavioral therapy compared with control conditions was associated with improved outcomes after treatment completion and at 1 to 6 months and at 6 to 12 months of follow-up for a generalized anxiety disorder (Hedges g, 0.07-0.40), panic disorder with or without agoraphobia (Hedges g, 0.22-0.35), social anxiety disorder (Hedges g, 0.34-0.60), specific phobia (Hedges g, 0.49-0.72), PTSD (Hedges g, 0.59-0.72), and OCD (Hedges g, 0.70-0.85). After 12-month follow-up, these associations were still significant for generalized anxiety disorder (Hedges g, 0.22; number of studies [k] = 10), social anxiety disorder (Hedges g, 0.42; k = 3), and PTSD (Hedges g, 0.84; k = 5), but not for panic disorder with or without agoraphobia (k = 5) and could not be calculated for specific phobia (k = 1) and OCD (k = 0). Relapse rates after 3 to 12 months were 0% to 14% but were reported in only 6 randomized clinical trials (predominantly for panic disorder with or without agoraphobia). Conclusions and Relevance: The findings of this meta-analysis suggest that cognitive behavioral therapy for anxiety-related disorders is associated with improved outcomes compared with control conditions until 12 months after treatment completion. After 12 months, effects were small to medium for generalized anxiety disorder and social anxiety disorder, large for PTSD, and not significant or not available for other disorders. High-quality randomized clinical trials with more than 12 months of follow-up and reported relapse rates are needed.