Tumor-to-tumor metastasis is a rare phenomenon defined as a metastasis in a histologically distinct tumor. We report an unusual case of a 51-yr-old woman who presented with a cervical, bleeding lesion diagnosed as a poorly differentiated adenocarcinoma, HPV18+. Imaging studies revealed an unresectable cervical carcinoma with an associated left ovarian multicystic teratoma. A suspicious solid nodule within the teratoma was also reported. Bilateral salpingo-oophorectomy was performed before the cervical tumor was treated. Pathologic findings showed a multicystic tumor measuring 110×40×30 mm with an area of adenocarcinoma, morphologically similar to the cervical neoplasm. The immunoprofile obtained in the malignant ovarian component (p16+, carcinogenic embryonic antigen+, vimentin−, estrogen receptor−) was similar to the one in the cervix. Moreover, both lesions harbored HPV18. These findings confirmed the presence of a metastatic cervical adenocarcinoma in a mature cystic teratoma. In the differential diagnosis, both somatic malignant transformation of the mature cystic teratomas and the presence of a “collision tumor” were considered and are discussed in this case report.