Signet ring cell carcinoma (SRCC) of the breast is a rare tumor that is classified into mucinous carcinoma and other tumors with abundant mucin as an independent subset and entity under the 2003 World Health Organization (WHO) classification of tumors of the breast. Pure form SRCC is very rare. To date, only a few cases have been reported in the literature. A case of primary SRCC of the breast in a 48-year-old woman is presented in this report. Histologically, the majority of the neoplastic cells are characterized by large intracytoplasmic mucin, compressing the nuclei toward one pole of the cell. The patient underwent both mastectomy and axillary lymph node excision. Adjuvant chemotherapy and chest wall irradiation were performed, and the case was discharged and followed up in remission for 20 months. The primary breast tumor was confirmed by clinical, morphological, and immunohistochemical studies. Characteristic features and differential diagnosis of this tumor were discussed in the light of pertinent literature.