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Objectives Cancer-related fatigue (CRF) is one of the most prevalent and debilitating symptoms among women undergoing neoadjuvant chemotherapy for breast cancer. This study analysed fatigue trajectories and identified modifiable determinants of fatigue worsening.Methods Prospective cohort of women with stage I–III breast cancer treated with neoadjuvant chemotherapy at a national cancer centre. Fatigue (Functional Assessment of Chronic Illness Therapy-Fatigue), clinical data, physical activity, handgrip strength, depressive symptoms, pain, insomnia and anaemia were assessed at baseline and across chemotherapy cycles. Generalised estimating equations modelled fatigue variation over time.Results Fatigue remained stable after anthracyclines but worsened significantly during the taxane phase. Independent determinants of greater fatigue worsening included overweight/obesity, insomnia, depressive symptoms, pain, anaemia and lower handgrip strength. Moderate-to-vigorous physical activity was protective.Conclusions Several modifiable factors contributed to CRF worsening during neoadjuvant treatment. Early identification and management of these conditions may mitigate fatigue progression and improve quality of life and treatment tolerance.