A case of a 36-yr-old woman with retroperitoneal synovial sarcoma is described. Her presenting symptom was epigastric pain that radiating to the back. On radiologic study, bulky retropancreatic soft tissue mass was detected which showed cystic and solid components. At operation, complete resection of the tumor was not possible because of the adhesion to the vena cava and the liver. During the follow-up, extensive tumor recurrence and liver metastasis were revealed. Primary retroperitoneal synovial sarcoma is a very rare malignant tumor with high mortality and recurrence rates. Retroperitoneal synovial sarcoma usually appears as a nonspecific soft tissue mass that do not have specific imaging features differentiating it from other mesenchymal tumors. However general radiologic findings and anatomic location of the tumor may help the diagnosis. In addition, synovial sarcoma should be included in the differential diagnosis of retroperitoneal soft tissue mass detected in young adults.