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Background: Ectopic breast tissue (EBT) develops along the mammary line owing to incomplete embryologic regression of the mammary ridges. This includes supernumerary breasts and aberrant breast tissue, in which malignancy is rarely reported. The commonest site of ectopic breasts is at the axilla, and the subcutaneous axillary mass may pose a diagnostic challenge to the clinician. Case Report: We report the case of a 31-year-old multiparous Nigerian woman who presented with a painless left axillary mass of two months' duration. The anatomical breasts were grossly and radiologically normal. A diagnosis of aberrant breast carcinoma in her left axilla was confirmed by tissue biopsy. She had wide local excision and left axillary dissection, followed by one course of cytotoxic chemotherapy before she began radiotherapy. Conclusion: Malignancy of aberrant breast tissue is a rare entity. A high index of suspicion and a low threshold for biopsy of subcutaneous lesions in the periphery of the breast allows for early intervention and a better prognosis. Triple assessment with clinical, radiological and pathological assessment of lesions in the axilla or along the embryonic milk line can not be overemphasized