Purpose <p>Hyperglycaemia and/or type 2 diabetes (T2D) can have a detrimental effect on women with breast cancer (BC) undergoing or post-cytotoxic chemotherapy. This systematic review aims to evaluate the short and long-term consequences of hyperglycaemia and/or T2D on treatment outcomes in women with breast cancer receiving cytotoxic chemotherapy.</p> Methods <p>Studies published between 2018 and 2024 across four electronic databases were identified. The JBI critical appraisal tool was adopted to select high-quality studies.</p> Results <p>Nine papers met the criteria for review. Thematic analysis identified two themes: 1) short-term consequences of hyperglycaemia and/or T2D, specifically its impact on healthcare utilisation, treatment toxicity, and treatment modification, and 2) long-term consequences of hyperglycaemia and/or T2D, such as effects on pathological response, prognosis, and mortality.</p> Conclusion <p>Proactive identification and rigorous management of hyperglycaemia and/or T2D are essential to reducing complications and improving outcomes in women with BC receiving chemotherapy. Evidence demonstrates that poor glycaemic control clearly impairs treatment response. The current research gap and fragmented care pathways demand strengthened multidisciplinary collaboration and the delivery of personalised care. These measures are necessary to significantly improve the quality of living with and beyond a diagnosis of BC.</p>