We report a rare case of primary small cell carcinoma of the breast. A 44-year-old woman was admitted to our hospital with a mass in her left breast. Fine-needle biopsy revealed small cell carcinoma with neuroendocrine differentiation resembling small cell carcinoma of the lung. Systemic computed tomography (CT) and magnetic resonance imaging (MRI) revealed no primary site in the lung or any other organ. A modified radical mastectomy with removal of the axillary lymph node (Bt + Ax, R2) was performed. Histological examination revealed that the tumor was composed of small round to oval cells with a large nuclear-cytoplasmic ratio. The tumor cells were positive for neuroendocrine differentiation markers such as synaptophysin, CD56, and neuron-specific enolase (NSE), but negative for thyroid transcription factor-1 (TTF-1), leukocyte common antigen (LCA), estrogen receptor (ER), and progesterone receptor (PR). Interestingly, the tumor cells lacked immunoreactivity for epithelial markers, including cytokeratin AE1/3, CAM5.2, and epithelial membrane antigen (EMA). The patient was given adjuvant chemotherapy for axillary lymph node metastasis. There were no signs of recurrence 22 months after surgery.