We report the first documented clinical case of the use of magnetic seeds to mark axillary lymph node metastasis in breast cancer before neoadjuvant chemotherapy. After chemotherapy, the patient showed a complete radiological response. One single sentinel lymph node was detected using a radiotracer, while the marked node was intraoperative magnetometer-guided identified. The analysis of the nodes showed negative sentinel lymph node and positive marked node, and the subsequent targeted axillary dissection was performed. Marking axillary positive lymph nodes with a magnetic seed is a simple and effective procedure for the intraoperative localisation of the node after neoadjuvant treatment.