Objective: The study aims to investigate the effectiveness of yoga on negative emotions in breast cancer patients. Methods: Pubmed, Elsevier, Web of Science, Cochrane Library, CBM, CNKI, Wanfang, and VIP databases were screened throughout October 2015. Randomized control trials (RCTs) examining the effects of yoga versus a non-exercise or waitlist control group on negative moods in breast cancer patients were included. The methodological quality of included RCTs was evaluated by using the Cochrane Handbook 5.1, and data were analyzed using the Review Manager 5.3. Results: A total of 21 RCTs with 1762 participants were included. We found evidence for immediate effects on anxiety (p < 0.00001), depression (p < 0.00001), distress (p < 0.00001), perceived stress (p < 0.00001), and emotional well-being (p = 0.0002). Sustained effects (3 months) were only found in depression (p = 0.004) but not anxiety (p = 0.43), and other outcomes were not synthesized because of heterogeneity and the limited number of studies. Conclusion: Yoga is valuable in improving negative moods in patients with breast cancer. We also concluded five key mechanisms of yoga therapy in improving negative moods. Further well-designed RCTs with large sample size and long-term follow-up are needed. Copyright (C) 2016, Chinese Nursing Association. Production and hosting by Elsevier B.V.