PURPOSE: This clinical trial aimed to evaluate the influence of Mat Pilates and time on the change in fatigue scores in women with breast cancer undergoing adjuvant radiotherapy (RT). Additionally, assess the adherence and adverse effects of Mat Pilates sessions and the association of the level of physical activity with severe fatigue symptoms. METHODS: One hundred fifty-six patients with non-metastatic breast cancer were randomized to usual care or supervised Mat Pilates exercise. Fatigue and physical activity level were measured at baseline, end of RT, 30 days, 3 and 6 months after RT. The generalized estimating equation (GEE) with intention to treat was applied. RESULTS: A significant difference in the fatigue mean between the end of RT and the baseline was found in patients of both groups. There was no adverse effect with the practice of Mat Pilates, and it was analyzed the reduction of the symptom pain after the end of RT in women of intervention group compared to the control. There was no significant effect on fatigue between the groups. Patients with severe fatigue after 3 and 6 months of RT reported a significantly lower level of physical activity in the last periods. CONCLUSION: Fatigue levels increased at the end of RT but returned to baseline values after 6 months. A lower level of physical activity was associated with severe fatigue symptoms. Mat Pilates was safe for these women and reduced the symptom pain after treatment, but it did not successfully reduce fatigue during adjuvant RT. REGISTRATION: NCT03333993. November 7, 2017.  https://clinicaltrials.gov/ct2/show/NCT03333993?term=breast+cancer&cond=pilates&draw=2&rank=1 .