Malignant mixed Müllerian tumor (MMMT) is a rare neoplasm of the female genital system. A case of MMMT in the uterus of an 8-year-old female dwarf rabbit, which died with clinical signs associated with severe acute dyspnea and anorexia, is described. At necropsy, an oval, firm tumor was found in each of the 2 uterine horns. Numerous metastases were scattered throughout the mediastinum, thoracic diaphragm, and all pulmonary lobes. Microscopically, the tumors consisted of a poorly demarcated, unencapsulated neoplasm, composed of closely associated carcinomatous and sarcomatous components and areas of osteosarcomatous differentiation. Metastases were composed entirely of the sarcomatous component with osteosarcomatous differentiation. Immunohistochemically, the neoplastic epithelial component was positive for cytokeratin and negative for α-smooth muscle actin (α-SMA), vimentin, and desmin. The sarcomatous component was diffusely and strongly positive for vimentin, focally positive for α-SMA (<20% of cells positive), and negative for desmin. The neoplasm was diagnosed as a heterologous MMMT with metastases to the lung, mediastinum, and thoracic diaphragm.