A seven-year-old entire female, mixed-breed dog, weighing 19 kg, was presented with a 2-day history of abdominal distension, reduced appetite and general dullness. Abdominal palpation revealed a large mass. Radiography showed a large mass occupying the left mid-abdominal area and convoluted loops of tubular fluid opacity occupying the right mid-abdominal area. Ultrasonography revealed a large heterogeneous mass with an anechoic area and some hyperechoic foci, indicative of calcification, in the mid-abdominal area. Furthermore, hypoechoic areas were found in the middle and caudal abdominal area and were presumed to be the fluid-filled uterine horns. At laparotomy, a 10.5x14.5-cm firm mass was found in the uterine body, while the uterine horns were filled with a thick red-brownish exudate; ovariohysterectomy was subsequently performed. A diagnosis of uterine leiomyosarcoma associated with pyometra was established by histopathology and immunohistochemistry.