Breast cancers that present with mucin include mucinous carcinoma and carcinoma with signet ring cell differentiation. The former shows extracellular mucin and the latter shows abundant intracellular mucin. Here, we report a case of breast cancer showing both extracellular mucin and extensive signet ring cell differentiation due to abundant intracellular mucin. Unlike mucinous carcinoma, this case had the features of high-grade nuclear pleomorphism, high mitotic index, estrogen receptor negativity, progesterone receptor negativity, human epidermal growth factor receptor-2 positivity, and ductal type with positivity for E-cadherin. In a case with signet ring cell differentiation, differential diagnosis with metastatic signet ring cell carcinoma of the stomach and colon is essential. In this case, the presence of accompanied ductal carcinoma in situ component and mammaglobin and gross cystic disease fluid protein-15 positivity were findings that suggested the breast as the origin.