We present a case of well-differentiated papillary mesothelioma discovered during staging surgery for endometrial carcinoma in a 50-year-old postmenopausal woman. In case of simultaneous well-differentiated papillary mesothelioma (WDPM) and endometrial carcinoma, the surgeon may be mistaken by considering peritoneal implants as tumor metastasis. This situation may result in overtreatment of the patient. Thus a thorough pathologic examination of the specimens taking care not to miss any areas of invasion, and utilizing immunohistochemical analysis when necessary are important to avoid such mistakes. To our knowledge this is the first report of the simultaneous occurrence of endometrial carcinoma in conjunction with diffuse WDPM of the peritoneum.