Breast carcinosarcoma is an extremely rare, clinically aggressive tumor, and no standard treatment has been established. We report about a 34-year-old woman presenting with a 2.5-cm-sized carcinosarcoma in her right breast. She presented to our hospital for examination of this mass. Ultrasonography showed a hypoechoic mass with partially irregular margins. Fine-needle aspiration cytology indicated malignancy. No enlarged lymph nodes or distant metastases were detected. We diagnosed right breast cancer and performed partial mastectomy, sentinel lymph node biopsy, and latissimus dorsi muscle flap transfer. Histological findings revealed that the tumor consisted of a mixture of an epithelial component and a mesenchymal component. The final diagnosis was carcinosarcoma. After undergoing adjuvant chemotherapy and radiotherapy, the patient has had no recurrence, and her cosmesis is maintained. Clinical data of carcinosarcoma are insufficient. Breast conservation and reconstruction for carcinosarcoma may be suitable as local treatments; however, the most appropriate treatment method has not been established.