Background: True carcinosarcoma of the breast is an extremely rare condition, accounting for 0.08–0.2% of all breast malignancies. The correct definition of this tumor requires both a carcinomatous component and a malignant non-epithelial component of mesenchymal origin, without evidence of a transition zone between the two elements. Case presentation: We present a case of a 49-year-old woman presenting with a 4 cm mass at the level of her left breast upper-outer quadrant with a histologic diagnosis of true carcinosarcoma of the breast. Discussion: The most appropriate therapeutic regimens for breast carcinosarcoma are still unclear because of the rarity of this condition, but Breast Conserving Treatment (BCT) followed by adjuvant chemotherapy seems to provide a prognosis equalling that of usual Invasive Ductal Carcinoma of the breast.