Clear cell carcinoma, which accounts for 10% of all ovarian cancers, has the poorest prognosis among ovarian cancer subtypes because of chemoresistance. Making a differential diagnosis between clear cell carcinoma and other ovarian epithelial tumors is important for determining the appropriate treatment modality. Napsin A, a member of the peptidase A1 family, is an aspartic proteinase that is expressed in normal lung and kidney tissues. We investigated the expression of napsin A in ovarian clear cell carcinoma using immunohistochemistry, and determined its usefulness for differentiating among primary ovarian epithelial tumors. A total of 36 ovarian cancer cases (16 primary clear cell carcinoma, 13 primary serous carcinoma, and 7 primary ovarian endometrioid carcinoma) with definitive diagnoses made between 1998 and 2015 were including in the study. All cases were evaluated for the immunoexpression of napsin A. In primary ovarian clear cell carcinoma cases, 3 (18.7%) stained negative for napsin A, whereas extensive strong cytoplasmic expression was observed in 13 (81.3%). No napsin A expression was detected in primary ovarian endometrioid and serous carcinoma samples. In conclusion, napsin A expression is a highly sensitive and specific marker that can be used to differentiate ovarian clear cell carcinoma from other tumors pathologically. It is expressed at high levels in ovarian clear cell carcinoma but not in other epithelial tumors.