IMPORTANCE: Mindfulness-based interventions (MBIs), grounded in mindfulness, focus on purposely paying attention to experiences occurring at the present moment without judgment. MBIs are increasingly used by patients with cancer for the reduction of anxiety, but it remains unclear if MBIs reduce anxiety in patients with cancer. OBJECTIVE: To evaluate the association of MBIs with reductions in the severity of anxiety in patients with cancer. DATA SOURCES: Systematic searches of MEDLINE, Embase, Cochrane Central Register of Controlled Trials, CINAHL, PsycINFO, and SCOPUS were conducted from database inception to May 2019 to identify relevant citations. STUDY SELECTION: Randomized clinical trials (RCTs) that compared MBI with usual care, waitlist controls, or no intervention for the management of anxiety in cancer patients were included. Two reviewers conducted a blinded screening. Of 101 initially identified studies, 28 met the inclusion criteria. DATA EXTRACTION AND SYNTHESIS: Two reviewers independently extracted the data. The Cochrane Collaboration risk-of-bias tool was used to assess the quality of RCTs, and the Preferred Reporting Items for Systematic Reviews and Meta-analyses reporting guideline was followed. Summary effect measures were reported as standardized mean differences (SMDs) and calculated using a random-effects model. MAIN OUTCOMES AND MEASURES: Our primary outcome was the measure of severity of short-term anxiety (up to 1-month postintervention); secondary outcomes were the severity of medium-term (1 to ≤6 months postintervention) and long-term (>6 to 12 months postintervention) anxiety, depression, and health-related quality of life of patients and caregivers. RESULTS: This meta-analysis included 28 RCTs enrolling 3053 adults with cancer. None of the trials were conducted in children. Mindfulness was associated with significant reductions in the severity of short-term anxiety (23 trials; 2339 participants; SMD, -0.51; 95% CI, -0.70 to -0.33; I2 = 76%). The association of mindfulness with short-term anxiety did not vary by evaluated patient, intervention, or study characteristics. Mindfulness was also associated with the reduction of medium-term anxiety (9 trials; 965 participants; SMD, -0.43; 95% CI, -0.68 to -0.18; I2 = 66%). No reduction in long-term anxiety was observed (2 trials; 403 participants; SMD, -0.02; 95% CI, -0.38 to 0.34; I2 = 68%). MBIs were associated with a reduction in the severity of depression in the short term (19 trials; 1874 participants; SMD, -0.73; 95% CI; -1.00 to -0.46; I2 = 86%) and the medium term (8 trials; 891 participants; SMD, -0.85; 95% CI, -1.35 to -0.35; I2 = 91%) and improved health-related quality of life in patients in the short term (9 trials; 1108 participants; SMD, 0.51; 95% CI, 0.20 to 0.82; I2 = 82%) and the medium term (5 trials; 771 participants; SMD, 0.29; 95% CI, 0.06 to 0.52; I2 = 57%). CONCLUSIONS AND RELEVANCE: In this study, MBIs were associated with reductions in anxiety and depression up to 6 months postintervention in adults with cancer. Future trials should explore the long-term association of mindfulness with anxiety and depression in adults with cancer and determine its efficacy in more diverse cancer populations using active controls.