An 11-year-old spayed female maltese was presented for abdominal distention, dysuria and dyschezia. Pantingand heart murmur was found and abdominal palpation was difficult due to increased abdominal pressure. A soft tissuemass, 6 × 3 cm in size, was identified radiographically in pelvic canal, displacing the descending colon to the medioventraldirection and the urinary bladder cranially. On ultrasonography, the mass consisted of homogeneous hypoechoicparenchyma containing the focal hyperechoic region (1.6 × 1.5 cm). The mass had distinct margin and no connectionwith adjacent organs. It was considered as a mass originating from the retroperitoneal cavity. Additional diagnosticprocedures were not advanced because of the owner’s request, and only a surgical excision of the mass was performedto alleviate the dysuria and dyschezia. Histopathologic examination and immunohistochemistry determined the massas a soft tissue sarcoma and excluded hemangiosarcoma and osteosarcoma, both are the most common types of theretroperitoneal tumors. This report described non-vascular soft tissue sarcoma originating from the retroperitoneal cavityin a maltese dog.