We report a case of cystadenocarcinoma arising in the retromolar area. A 68-year-old male was referred with a painless, ulcerative, well defined, proj ecting, elastic-firm, and moderately mobile mass of the right retromolar area. Incisional biopsy was performed, and the pathological diagnosis of the specimen was sialadenoma papilliferum. Excision of the tumor was performed under general anesthesia. The histopathological diagnosis was cystadenocarcinoma, showing a cystic tumor that featured papillary proliferation of neoplastic cells varying from cuboidal to columnar. Postoperative external irradiation and chemotherapy were performed, because infiltrative growth and lymph vessel invasion were found in the circumference. There was no recurrence during 20 months of follow-up.