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Introduction During neoadjuvant chemotherapy (NACT), patients with breast cancer (BC) often experience physical discomfort, fatigue, sleep disturbances, and psychological stress that negatively affect physical functioning. While exercise interventions may improve physical fitness and overall health, limited evidence exists on the relationship between weekly health symptoms and fitness outcomes of high-intensity aerobic interval training (HIIT) implemented during NACT.Methods Thirty-five female participants (mean age = 48.6 ± 7.8 years) with newly diagnosed stage II–III BC undergoing NACT were randomly assigned to HIIT group (n = 17) and control group (CG; n = 18). The HIIT consisted of 4 × 4 min intervals at 85–95% HRpeak with 3 min active recovery at 70% HRpeak, three times/week. All participants completed a weekly online survey assessing physical and psychological complaints, fatigue, social activity, and chemotherapy side effects.Results From baseline to post-test the HIIT group maintained no significant changes in VO2peak (p=0.943), HRmax (p = 0.113) and RER (p = 0.068), while CG demonstrated significant declines in VO2peak (p < 0.001), HRmax (p = 0.050), TTE (p = 0.003), and speed (p = 0.002). Higher VO2peak in the HIIT group was associated with better weekly physical functioning (p < 0.001) and psychological state (p = 0.007). HIIT group improved sleep quality throughout the NACT (p = 0.003).Conclusions HIIT group during chemotherapy maintained aerobic capacity and was associated with better physical functioning, psychological well-being, and sleep quality compared with CG, highlighting the importance of weekly health monitoring in HIIT based BC treatment.