Neoadjuvant chemotherapy (NACT) is increasingly used in breast cancer. The best proof of NACT efficacy is pathological complete response (pCR), i.e. the absence of invasive tumour on post-NACT surgical histopathology. While it is known that physical exercise can help patients to better tolerate and complete often harsh cancer treatments, it is an emerging area of research to understand if and how exercise exerts anti-tumour effects and improves oncological outcomes. The main aim of the Neo-ACT trial is to examine if a physical exercise intervention during NACT can increase pCR rates in breast cancer. Secondary aims are residual cancer burden, radiological tumour response, patient-related outcomes (health-related quality of life, physical activity), physiological outcomes (muscle strength, cardiorespiratory fitness), cancer treatment-related toxicities (cognitive dysfunction, chemotherapy completion rates) and long-term sick leave. Furthermore, the trial will explore how physical exercise affects anti-tumoral mechanisms inherent to therapy or host by hypothesis-generating translational analyses. 712 patients with primary invasive breast cancer will be randomized to either a supervised intervention of high-intensity interval and resistance training during NACT, supported by an exercise app, or to usual care, and followed for two years. Physical activity is meticulously tracked. By offering patients active involvement, the trial contributes strongly to the concept of personalized medicine.