Background: Scarce evidence exists on barriers to physical activity in Mexican women. Despite evidence from other countries, no research has investigated the influence of the breast on PA in this population. Objective: To determine the association between the breast and physical activity in Mexican women. Design: Cross-sectional observational study. Methods: Volunteers were 279 Mexican women from Veracruz, Durango, and Baja California states, who completed a paper survey of their demographics, brassiere characteristics, breast pain, and frequency and amounts of weekly physical activity. Results: The first barrier to physical activity was time constraints, followed by breast-related issues. Breast pain was reported by 47.1% of women, and the breast as a barrier to physical activity participation was reported by 30.6%. Responses, such as “I am embarrassed by excessive breast movement” and “My breasts are too big” were the most frequently reported breast-related barriers to physical activity. Breast pain was associated with the menstrual cycle and exercise. Breast health knowledge and pain intensity were unrelated to moderate- and vigorous-intensity physical activity. The 36.4% and 6.7% of women did not meet weekly moderate- and vigorous-intensity physical activity guidelines, respectively. Weekly moderate- and vigorous-intensity physical activity was similar between women reporting breast pain and those who did not. Conclusions: Because the breast was the second most significant barrier to physical activity, it is imperative to increase breast health knowledge in Mexican women to reduce impediments to physical activity.