Malignant mixed Müllerian tumor (MMMT) is a rare neoplasm of the female genital tract. We report a case of bilateral ovarian MMMT in a 10-year-old female dog. Ovaries were only moderately enlarged with a papillary surface and firm nodules. Multiple metastases were observed in the abdominal cavity and pulmonary parenchyma. Histologically, both ovaries had intermingled carcinomatous and sarcomatous components with cartilage and bone. Metastatic lesions were not mixed. The peritoneal metastases were carcinomatous; pulmonary metastases were sarcomatous. Carcinomatous elements of the MMMT were immunohistochemically positive for cytokeratin (CK) AE1/AE3, CK7, and vimentin and estrogen receptors. Conversely, the sarcomatous cells were positive for vimentin, but negative for CKs. Chondrocytes also expressed S-100 protein. On the basis of similarities to human ovarian MMMT, the diagnosis was heterologous malignant mixed Müllerian tumor of the ovary.