Synovial Sarcoma (SS) is a rare soft-tissue malignancy. Only about 15% of SS originates from the retroperitoneum. Retroperitoneal SS (RSS) is usually diagnosed incidentally due to the anatomy of the retroperitoneum. The most common complaints of patients are abdominal and low back pain. Other common symptoms of RSS are palpable abdominal mass, weight loss, and anemia. In this study, we will describe a 29-year-old white Asian man with a diagnosis of RSS after radical nephrectomy. He was admitted to the Urology Department of Shohada-e Tajrish hospital, Tehran, Iran in March 2019. The distinguishing feature of this case is the size of the mass, which has never been reported so much for retroperitoneal synovial sarcoma. Radiologic imaging showed a huge retroperitoneal mass originating from the kidney. Based on the pathologic features and immunohistochemistry (IHC) study, the diagnosis was consistent with synovial sarcoma. Accurate diagnosis of RSS is usually based on the pathological findings. Therefore, in case of doubt, a biopsy can be employed. Surgical resection of the tumor and lymph nodes dissection is the main and most important part of the treatment. Aggressive resection with free margin is recommended. The role of adjuvant and neoadjuvant chemotherapy in RSS is not certain to date, but it is recommended according to the patient’s condition.