This report describes a rare case of invasive extramammary Paget disease of the vulva with signet ring cell morphology in a 58-yr-old woman with a history of signet ring cell carcinoma of the stomach. This case was initially misinterpreted as a metastatic gastric carcinoma to the vulva because an initial small, superficial biopsy specimen showed infiltration of signet ring cells in the dermis without intraepidermal Paget cells. However, a surgically resected specimen showed concordant immunophenotypes in both intraepidermal Paget cells and intradermal signet ring cell components with immunoreactivity to cytokeratin (CK) 7, CEA, and gross cystic disease fluid protein-15, and immunonegativity for CK20, MUC5AC, and MUC6. Gastric signet ring cell carcinoma showed immunoreactivity to CK7, CEA, MUC5AC, and MUC6, and immunonegativity for gross cystic disease fluid protein-15 and CK20. The diagnosis of primary invasive extramammary Paget disease of the vulva was also supported by a long interval after gastrectomy (7.5 yr), the solitary involvement of the vulva, and the absence of lymphovascular invasion. This case demonstrates that invasive extramammary Paget disease may have a signet ring cell morphology and immunohistochemical profile similar to those of gastric signet ring cell carcinoma, but the addition of gross cystic disease fluid protein-15 immunostain in the panel of markers is helpful in the differential diagnosis.