We report a case of primary mucinous cystadenocarcinoma of the vermiform appendix. A 54-year-old woman presented to a local clinic with a chief complaint of right lower abdominal pain. A right ovarian tumor was suspected, and the patient was referred to our gynecology department. Surgery was performed for a preoperative diagnosis of right ovarian tumor, but laparotomy revealed a primary tumor of the appendix, and appendectomy was performed instead. The intraoperative pathological examination did not reveal any evidence of malignancy, however, the final histological diagnosis was primary mucinous cystoadenocarcinoma of the appendix, and right hemicolectomy with lymph node dissection (D3) was performed two weeks after the initial operation. The tumor was histological stage II (p0, h0, n0, and ss).Primary mucinous cystadenocarcinoma of the appendix is rare and difficult to differentiate from appendicitis and other diseases preoperatively.