Background Several cognitive behavioral interventions have been reported to reduce psychological symptoms in breast cancer (BC) patients. The goal of this study was to evaluate the effects of a yoga intervention in reducing depression and anxiety symptoms in BC patients. Methods This study was a randomized, partially blinded, controlled trial comparing a standardized yoga intervention to standard care. It was conducted at three medical centers in Montreal, Canada. Eligible patients were women diagnosed with stage I-III BC receiving chemotherapy. Participants were randomly assigned to receive yoga intervention immediately (experimental group, n=58) or after a waiting period (n=43 control group). The Bali Yoga Program for Breast Cancer Patients (BYP-BC) consisted of 23 gentle Hatha asanas (poses), 2 prayanamas (breathing techniques), shavasanas (relaxation corpse poses) and psychoeducational themes. Participants attended eight weekly sessions lasting 90 min each and received a DVD for home practice with 20- and 40-min sessions. Participants in the wait list control group received standard care during the 8-week waiting period. Results A total of 101 participants took part in the final intention-to-treat analyses. The repeated measures analyses demonstrated that depression symptoms increased in the control group (p=0.007), while no change was reported in the BYP-BC group (p=0.29). Also, depression symptoms decreased in the WL control group after receiving the BYP-BC intervention (p=0.03). Finally, there was no statistical significance in terms of anxiety symptoms (p=0.10). Conclusions Results support the BYP-BC intervention as a beneficial means of reducing and preventing the worsening of depression symptoms during chemotherapy treatment.