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Introduction: Despite strong evidence of the benefits of physical activity on reducing the side effects of breast cancer and associated treatments, many patients tend to be physically inactive. Understanding the facilitators and barriers to their engagement in physical activity and their continuation of it represents a first step toward designing targeted interventions to increase this activity. The aim of this study is to describe the determinants of the physical activity behavior of patients participating in the RIPOSTE program. Method: A qualitative study was conducted with ten patients undergoing invasive breast cancer surgery and participating in an adapted physical activity program based on fencing (RIPOSTE). Semi-structured interviews explored their physical activity experience and were analyzed both inductively and deductively using the Theoretical Domains Framework, examining mechanisms of behavior change. Results: Fifty-six themes were identified as facilitators and barriers in twelve of the fourteen domains. The “Environmental Context and Resources” and “Social Influences” domains were the most predominant. The “Social/Professional Role and Identity,” “Optimism,” and “Reinforcement” domains were not mentioned by all participants. Conclusion: Use of the Theoretical Domains Framework enabled the study to highlight the complexity of physical activity behavior change among cancer survivors. Our results provided a useful theoretical basis to guide the selection of modifiable factors when designing interventions to improve patients’ physical activity levels.