The patient was a 53-year-old woman in whom ultrasonography of the breast revealed a lobular mass, 14 mm in diameter, in the right AB region. Spindle cells were obtained on fine-needle aspiration biopsy, but it was not possible to diagnose whether the tumor was benign or malignant. Contrast-enhanced magnetic resonance imaging showed a mass with a cystic component that was darkly stained in the early phase. Needle biopsy showed a dense proliferation of atypical spindle cells with no distinct epithelial-like arrangement. The differential diagnosis included mesenchymal malignant tumors such as fibrosarcoma, some phyllodes tumors, and epithelial tumors with sarcomatoid differentiation. Immunostaining revealed that the tumor was cytokeratin (AE1/AE3)-negative, partially CAM 5.2-positive, p63-positive, S100-negative, SMA-positive, partially vimentin-positive, with a Ki-67 index of 80% and negativity for ER, PgR, and HER2. Spindle-cell carcinoma was thus diagnosed. A partial right mastectomy with sentinel lymph-node biopsy was performed. Immunostaining of the resected specimen confirmed spindle cell carcinoma. The General Rules for Clinical and Pathological Recording of Breast Cancer classify spindle cell carcinoma as a special type of invasive cancer with a sarcomatoid structure, consisting of spindle-shaped cancer cells. This type of carcinoma is extremely rare, accounting for less than 1% of all breast cancers.