Accessory or ectopic breast tissue is an anomaly in the development of the breast. It is a rare condition that occurs along the embryological mammary line. In less than 1% of all breast cancers, supernumerary breast cancer is reported, with the axillary location being the most common in 60% to 90% of cases. Cancerous degeneration of this supernumerary breast tissue can pose a dual diagnostic and therapeutic problem. We report the case of locally advanced adenocarcinoma in a right supernumerary breast. This is a 75-year-old, grand-multiparous, postmenopausal, and known hypertensive patient on treatment. Family history was remarkable for brain cancer in her sister and oesophageal cancer in her mother. She consulted for a mass in the right axillary cavity on supernumerary breast evolving for a year. Clinical examination revealed a large, fixed, budding and haemorrhagic-ulcerated mass of the right axilla, with long axis measuring about 15 cm. There was as wella supernumerary breast on the left, but without particularity. A soft tissue ultrasound showed a large hypoechoicmass in the right axillary region of 116 mm with areas of central necrobiosis. Morphologically, the breasts were normal. A breast MRI revealed a subcutaneous mass in the right axillary cavity with skin ulceration and satellite lymphadenopathy. The extension assessment revealed liver metastases, and a biopsy of the mass revealed a breast adenocarcinoma. The case was the subject of a multidisciplinary consultation meeting following which a wide excision of the mass was indicated. The histo-pathology analysis results of the surgical specimen were in favour of a triple negative How to cite this paper: