We report on a very rare case of peritoneal clear cell adenocarcinomas. A 49-year-old Japanese woman underwent hysterectomy and bilateral salpingo-oophorectomy for endometrial endometrioid adenocarcinoma grade III, which was composed of undifferentiated carcinoma cells (98%) and tubular carcinoma cells (2%). No clear cell adenocarcinoma elements were noted in this tumor. Two peritoneal cystic tumors were detected by imaging modalities around the stomach and spleen, 15 months and 21 months after the follow-up period of the endometrial carcinoma, respectively. These two tumors were surgically resected. They were cystic tumors encapsulated by fibrous capsules and showed the same morphologies. They showed proliferation of carcinoma cells arranged in solid nest, tubular, and papillary patterns. They showed clear cytoplasm positive for periodic acid-Schiff stain, hobnail cells, and occasional hyaline globules. The morphologies fulfilled the criteria of clear cell adenocarcinoma. The morphologies and immunohistochemical findings of the two peritoneal clear cell adenocarcinomas were different from those of endometrial carcinoma. We believe that the two clear cell adenocarcinomas are not metastatic lesions from the endometrial carcinoma of the uterus, and that they are primary clear cell adenocarcinomas of the peritoneum. Our case was characterized by cyst formations and encapsulation in addition to the common histological features of clear cell adenocarcinoma of the uterus and ovary.