BACKGROUND : Accessory or ectopic breast tissue is an aberration of normal breast development. It is known to be a rare entity present anywhere along the embryologic mammary streak or milk line but more common in axilla. METHODS : We report a case of 36 year old female with accessory breast carcinoma who presented with a progressive lump in her left axilla for 1 year. On examination a 2 cm solitary mass was palpable in axilla. Ultrasound confirmed a 19 mm mass with no other lesion in breast and axilla. Core biopsy showed invasive ductal carcinoma. She was discussed in multidisciplinary board meeting and was offered upfront surgery with excision of accessory breast tissue and sentinel lymph node biopsy. Axillary lymph node dissection was omitted following ACOSG Z0011 criteria. She was offered adjuvant chemotherapy and radiation post operatively along with endocrine treatment as she was hormone receptor positive. CONCLUSIONS : Accessory breast development is hormone dependent just like normal breast. Breast cancer in accessory breast tissue is very rare. The incidence is around 6%. Most common pathology is invasive ductal carcinoma (50-75%). The most common location is axilla (60-70%) although it can present in other less common locations like infra-mammary region (5-10%) and rarely in thighs, perineum, groin, and vulva. CONCLUSIONS : Since accessory axillary breast tissue is out of the image of screening breast examination, it is necessary for the oncologists to be aware of this entity and associated pathologies. Their preventive excision in high risk women can also be considered.