An atypical polypoid adenomyoma (APAM) is a benign mixed epithelial and mesenchymal uterine tumor. Patients typically present with hypermenorrhea or abnormal uterine bleeding, and the tumor is most commonly found in nulliparous or infertile women of reproductive age. The natural pathological course of an APAM remains unclear because of the rarity of the disease. The coexistence or sequential development of low-grade malignant endometrial tumors has been reported; however, reports of a mixed carcinoma arising from an APAM have not been published yet. In this report, the authors describe the case of a woman with a mixed carcinoma of the uterus arising from an APAM. This case extends our knowledge of the natural pathological course of an APAM and its etiological relationship with a mixed carcinoma of the uterus, which may contribute to the development of management strategies for an APAM.