BACKGROUND Primary breast small cell neuroendocrine carcinoma is a rare subtype of breast cancer with about 57 cases reported in the literature. This rare type of cancer represents about 0.1% of breast carcinomas. Recently, the World Health Organization defined this type of cancer as a separate entity from other breast cancer types. The diagnosis of this type of cancer in the breast is difficult because the histological pattern is similar to the small cell neuroendocrine carcinoma of other more common primary sites of origin, including the lung. CASE REPORT A 39-year-old woman presented to our hospital with a left breast mass and recurrent mastitis. Physical examination revealed a painless lump in her left breast with a brown-colored discharge from the nipple, and her child refused breastfeeding from the left breast. A high-density well-defined rounded mass was observed upon mammography in the upper lateral aspect of the left breast. This mass lesion appeared hypoechoic with no posterior acoustic shadowing on ultrasound scan. A core-needle biopsy of the mass was performed and the diagnosis of small cell neuroendocrine carcinoma was rendered after histopathologic examination. Positron emission tomography scanning was helpful in the exclusion of primary origin from other organ sites; thus, the primary breast origin of the tumor was confirmed. CONCLUSIONS This case report provides a comprehensive approach to diagnose this type of small cell carcinoma originating primarily in the breast. The suspicion of this type of breast cancer should be raised if there is presence of characteristic histopathologic findings with the exclusion of any primary origin from other organ sites by the help of imaging studies.