Alveolar Soft Part Sarcoma (ASPS), also called Alveolar Soft-Tissue Sarcoma, is a rare type of soft-tissue neoplasm with a poor long term prognosis. Such tumors originating in the retro peritoneal space are extremely rare. In this article we discuss a 34-year-old woman who was referred to our hospital with an increasing mass in her left lower abdomen. Ultrasonography and conventional Computed Tomography revealed a large hard mass occupying the left retroperitoneal space with a clear border. The pathological diagnosis was ASPS.