Synovial sarcoma (SS) usually affects joints, bursae, and tendons of extremities and is very infrequent in the head and neck, abdomen, thorax, prostate and kidney, skin, blood vessels, and nerves. Primary intra-abdominal SS is exceeding uncommon and has non-specific symptoms or compress surrounding structures. The diagnosis is a challenge, and histopathological and immunohistochemical studies must confirm the hypothesis. We report the case of SS that has origin in peritoneal structures and a longstanding unsuspected course. The patient was a 64-year-old woman who claimed chronic pain in the left iliac fossa, without additional symptoms. She related laparoscopic oophorectomy, cholecystectomy, and abdominal hysterectomy in the previous three decades. There was neither local invasion nor lymph nodal, vascular or neural invasion, and her surgical treatment by open abdominal procedure was uneventful. The herein reported case aims to enhance the index of suspicion.