A six-year-old male castrated mixed breed dog presented to Cornell University for acute presentation of a left abdominal mass. The patient had become lethargic with a distended abdomen after playing with another dog. The lethargy resolved over 48 hours, but the abdominal distension remained. Abdominal palpation revealed a large, non-painful left cranial to mid-abdominal mass. An abdominal ultrasound and abdominal radiographs could not discern the origin of the mass. Thoracic radiographs were negative for pulmonary metastases. An exploratory abdominal surgery was performed, which revealed a large mass encompassing the left kidney and a focal splenic lesion. A nephrectomy, splenectomy, prophylactic gastropexy, and lymph node biopsy were performed. Histology revealed renal sarcoma, focal splenic necrosis, and a reactive local lymph node. Immunohistochemistry revealed a smooth muscle sarcoma with the two primary differentials as hemangiopericytoma and myofibrosarcoma. Renal sarcomas are rare and aggressive cancers and precise histogenesis can be difficult to determine.