Background: Primary poorly differentiated adenocarcinoma of the accessory breast is rare. We report such a case in a 56-year-old Malaysian woman of Indian ethnicity. Case Presentation: An axillary swelling measuring 5 × 5 cm on excisional biopsy revealed a poorly differentiated adenocarcinoma with positive HER2 gene amplification. Conclusion: Overall prognosis is similar to that of a carcinoma of the normal breast, therefore early diagnosis is imperative to ascertain curable status and improve prognosis.