IMPORTANCE: Anxiety disorders are common, highly distressing, and impairing conditions. Effective treatments exist, but many patients do not access or respond to them. Mindfulness-based interventions, such as mindfulness-based stress reduction (MBSR) are popular and can decrease anxiety, but it is unknown how they compare to standard first-line treatments. OBJECTIVE: To determine whether MBSR is noninferior to escitalopram, a commonly used first-line psychopharmacological treatment for anxiety disorders. DESIGN, SETTING, AND PARTICIPANTS: This randomized clinical trial (Treatments for Anxiety: Meditation and Escitalopram [TAME]) included a noninferiority design with a prespecified noninferiority margin. Patients were recruited between June 2018 and February 2020. The outcome assessments were performed by blinded clinical interviewer at baseline, week 8 end point, and follow-up visits at 12 and 24 weeks. Of 430 individuals assessed for inclusion, 276 adults with a diagnosed anxiety disorder from 3 urban academic medical centers in the US were recruited for the trial, and 208 completed the trial. INTERVENTIONS: Participants were 1:1 randomized to 8 weeks of the weekly MBSR course or the antidepressant escitalopram, flexibly dosed from 10 to 20 mg. MAIN OUTCOMES AND MEASURES: The primary outcome measure was anxiety levels as assessed with the Clinical Global Impression of Severity scale (CGI-S), with a predetermined noninferiority margin of -0.495 points. RESULTS: The primary noninferiority sample consisted of 208 patients (102 in MBSR and 106 in escitalopram), with a mean (SD) age of 33 (13) years; 156 participants (75%) were female; 32 participants (15%) were African American, 41 (20%) were Asian, 18 (9%) were Hispanic/Latino, 122 (59%) were White, and 13 (6%) were of another race or ethnicity (including Native American or Alaska Native, more than one race, or other, consolidated owing to low numbers). Baseline mean (SD) CGI-S score was 4.44 (0.79) for the MBSR group and 4.51 (0.78) for the escitalopram group in the per-protocol sample and 4.49 (0.77) vs 4.54 (0.83), respectively, in the randomized sample. At end point, the mean (SD) CGI-S score was reduced by 1.35 (1.06) for MBSR and 1.43 (1.17) for escitalopram. The difference between groups was -0.07 (0.16; 95% CI, -0.38 to 0.23; P = .65), where the lower bound of the interval fell within the predefined noninferiority margin of -0.495, indicating noninferiority of MBSR compared with escitalopram. Secondary intent-to-treat analyses using imputed data also showed the noninferiority of MBSR compared with escitalopram based on the improvement in CGI-S score. Of patients who started treatment, 10 (8%) dropped out of the escitalopram group and none from the MBSR group due to adverse events. At least 1 study-related adverse event occurred for 110 participants randomized to escitalopram (78.6%) and 21 participants randomized to MBSR (15.4%). CONCLUSIONS AND RELEVANCE: The results from this randomized clinical trial comparing a standardized evidence-based mindfulness-based intervention with pharmacotherapy for the treatment of anxiety disorders found that MBSR was noninferior to escitalopram. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03522844.