We report a case of a 45-year-old Japanese woman with adenoid squamous cell carcinoma (ASCC) of the left breast skin. The patient had showed a large mass in the left breast region with axillary swelling about 1 year before admission. Grossly, the tumor was an extensively ulcerated and elevated lesion measuring 15X16X5 cm. Based on the tumor biopsy and cytologic examination of the axillary lymph nodes, squamous cell carcinoma (SCC) was diagnosed. No evidence of distant metastasis was identified. A modified radical mastectomy with left axillary node dissection was performed. Microscopically, the resected tumor showed an invasive proliferation of atypical squamous cells with marked keratinization. At the periphery of the tumor, an adenoid growth pattern was frequently seen with a transitional area showing squamous cell carcinoma and adenoid growth components. ASCC was diagnosed. A transition between the overlying squamous cell epithelium and squamous cell carcinoma component was also seen, thus the tumor was thought to have originated from the breast skin. The patient died of respiratory failure due to multiple lung metastasis about 1 month after the mastectomy. Tumor rarely originates at the breast region to include both the mammary glands and breast skin. The pathogenesis and management of ASCC are discussed following the presentation of this case.