Patients with anxiety disorders need treatment options beyond psychotropic medications, which are sometimes not effective or preferred. Mindfulness-based interventions have become more acceptable in the United States and have been shown to reduce anxiety compared to treatment as usual. A prospective randomized, 2-arm parallel group controlled single-blinded clinical trial investigated whether a mindfulness-based stress reduction (MBSR) intervention was noninferior to escitalopram. Participants (18–75 years old) with anxiety disorders (generalized anxiety disorder, social anxiety disorder, panic disorder, or agoraphobia determined with structured psychiatric diagnostic interview) completed eight weeks of MBSR (n=102) or escitalopram treatment (10–20 mg flexibly dosed; n=106). MBSR was taught by qualified instructors and included 2.5-hour long classes, a day-long retreat weekend class during the fifth or sixth week, and 45 minutes of daily home practice exercises. The primary outcome measure was the Clinical Global Impression-Severity (CGI-S) scale for anxiety, assessed by trained clinicians. Clinical severity of anxiety at baseline was in the moderate to markedly ill range and did not differ by treatment group. At the study endpoint, the mean CGI-S score was reduced by 1.35 (1.06) for MBSR and 1.43 (1.17) for escitalopram; the difference between groups was not statistically significant (-0.07 [0.16], 95% confidence interval, -0.38 to 0.23; p=.65) indicating noninferiority of MBSR. At least one study-related adverse event occurred for 78.6% and 15.4% of participants randomized to escitalopram and MBSR, respectively (p<.001). A similar percentage of participants completed the MBSR (75%) and escitalopram (76.5%) interventions, but greater percentages of individuals continued with escitalopram versus MBSR at 12-week (78% vs. 49%) and 24-week (52% vs. 28%) follow-up assessments. Overall, this clinical trial found that MBSR intervention is noninferior to escitalopram, an established pharmacological treatment for anxiety disorders, and has fewer adverse effects associated with its use. These findings indicate that MBSR is an evidence-based treatment option for adults with anxiety disorders.