Background: In the last two decades, physical training has been recognized as an important means of prevention and rehabilitation for patients with breast cancer. In Brazil, physical exercise for patients with breast cancer is a new area of study. We aimed to investigate the feasibility of implementing a resistance exercise program for patients undergoing breast cancer treatment and to examine the effects of the resistance exercise program on psychological and physical outcomes. Methods: We conducted a non-randomized controlled trial. Eleven women (55.08 ± 7.8 years) undergoing radiotherapy and hormone therapy for breast cancer were divided into exercise group (n = 6) and control group (n = 5). The feasibility was evaluated by training load performed by each patient compared to the total training load prescription. The resistance exercise program consisted of 12 weeks and 36 sessions (3x per week), and intensity between 50 and 70% of one maximum repetition. Fitness (6-minute walk test), quality of life (EORTC QLQ-C30), mood states (BRUMS), sleep quality (Pittsburgh), and body image (Body Image after Breast Cancer) were investigated before and after resistance exercise program. Data were analyzed by comparison inter and intra groups (p=0.05). Findings: The resistance exercise program implementation was feasible (>80%). There were no adverse events due to the resistance exercise program, and the adherence averaged 83.6% with no dropouts. Twelve weeks of resistance exercise appeared to have positive effects on quality of life, sleep quality, physical fitness, body image, and fatigue in patients undergoing breast cancer treatment. Conclusions: The implementation of a resistance exercise program in a university in the south of Brazil proved to be feasible, tolerable,