Purpose <p>Recent advancements in medical technologies have made trans-arterial treatment of breast cancer feasible. Consequently, understanding the vascular anatomies of breast cancers and axillary lymph node metastases has become indispensable for sophisticated treatments. The aim of this study was to determine the vascular anatomy of the breast, which is crucial for trans-arterial chemoembolization in patients with breast cancer.</p> Materials and Methods <p>A total of 126 treatment-naive breast cancer patients were investigated to evaluate the arteries supplying breast tissues, tumours, and axillary lymph node metastases. The breast-supplying arteries were identified via angio-CT scans performed in the same examination room.</p> Results <p>As described in anatomical textbooks, the main arteries supplying breast tissues and tumours are mammary branches arising from the internal thoracic artery and lateral thoracic artery. However, an undescribed serpiginous artery arising from the brachial artery distal to the subscapular artery and circumflex humeral artery was discovered in 46.0% of patients. This artery partially supplied breast tumours in 30.2% of the patients. The circumflex mammary artery is presumed to be a normal variant that is not pathologically enlarged due to tumour angiogenesis. Axillary lymph node metastases are supplied mainly by the lateral thoracic artery and thoracodorsal artery.</p> Conclusion <p>We identified an undescribed serpiginous artery arising from the brachial artery distal to the subscapular artery or circumflex humeral artery, and we named this the circumflex mammary artery.</p> Graphical Abstract <p></p>