Spindle cell carcinoma of the breast is a rare tumor. This tumor can proliferate rapidly and cause cystic changes because of internal tissue necrosis. We evaluated a 54-year-old woman with right breast lump. Mammography showed a category four mass with a diameter of 2.5 cm. Ultrasonography(US) revealed a complex cystic lesion, and fine-needle aspiration(FNA) cytology demonstrated bloody fluid and malignant cells. Partial breast resection and sentinel lymph node biopsy were performed. Immunohistology revealed spindle cells with positive results for cytokeratin(AE1/AE3) and vimentin, partially positive results for s-100, and negative results for desmin and α-actin. The pathological stage was IIA, and biochemical characterization showed that the tumor was triple negative. Six courses of FEC-100 chemotherapy(5-fluorouracil 500 mg/m2, epirubicin 100 mg/m2, and cyclophosphamide 500 mg/m2) were administered. Radiotherapy was performed. This case is discussed with reference to the literature. This tumor can proliferate rapidly and cause cystic changes because of internal tissue necrosis. We evaluated a 54-year-old woman with right breast lump. Mammography showed a category four mass with a diameter of 2.5 cm. Ultrasonography (US) revealed a complex cystic lesion, and fine-needle aspiration (FNA) cytology demonstrated bloody fluid and malignant cells. Partial breast resection and sentinel lymph node biopsy were performed. Immunohistology revealed spindle cells with positive results for cytokeratin ( AE1 / AE3) and vimentin, partially positive results for s-100, and negative results for desmin and α-actin. The pathological stage was IIA, and biochemical characterization showed the the tumor was triple negative. Six courses of FEC-100 chemotherapy ( 5-fluorouracil 500 mg / m2, epirubicin 100 mg / m2, and cyclophosphamide 500 mg / m2) were administered. Radiotherapy was performed. This case is discussed with reference to the literature.