Primary mucinous adenocarcinomas of the uterine corpus are typically low grade and frequently associated with endometrial hyperplasia and/or ordinary endometrioid adenocarcinoma, but may appear as a heterogeneous group of neoplasms. A case is described of a 56-year-old postmenopausal woman who presented with mucinous vaginal discharge. Imaging demonstrated thickened myometrium due to adenomyosis. Serum CA19-9 levels were elevated to 486 U/mL. Microscopic examination of hysterectomy specimens indicated highly differentiated mucinous adenocarcinoma diffusely infiltrating the portion of adenomyosis of the corpus. In some areas endometrial glands of adenomyosis were replaced by benign-looking mucinous metaplasia. The uterine cervix showed no abnormalities. HIK1083 and MUC6 immunohistochemistry indicated a gastric phenotype of the tumor, as seen in cases of prototypical minimal deviation adenocarcinoma (MDA) of the cervix. In summary, mucinous endometrial adenocarcinoma rarely shows features similar to MDA of the cervix. This case provokes a discussion on diagnostic and management strategy, and histogenesis of mucinous neoplasm of the endometrium.