Gossypiboma (retained surgical sponge) is a pseudotumor within the body that is composed of non-absorbable surgical material with a cotton matrix. Because the symptoms of gossypiboma usually are nonspecific and may appear years after surgery, the diagnosis of gossypiboma may be difficult because the condition may mimic a benign or malignant soft-tissue tumour in the abdomen and pelvis. A 61-year-old woman with a one-year history of left upper-quadrant pain and weight loss was referred to our center. She had undergone peptic ulcer perforation 23 year ago. Physical examination revealed dullness and palpable mass in the left upper abdomen. On examination by computed tomography (CT), a hypodense mass of 12 cm in diameter between the greater curvature of the stomach, pancreas, and splenic hilus was detected. Upon exploration, a mass lesion of 10 cm in diameter was detected between the greater curvature of the stomach and splenic hilus, which caused dense adhesions not in communication with the pancreas. It was excised and a splenectomy was performed. After a macroscopic examination, the mass lesion was diagnosed as gossypiboma. Although ultrasonography (US), CT, angiography, and magnetic resonance imaging (MRI) may be used to diagnose gossypiboma, definitive diagnosis is possibile only upon surgery or histopathological examination. As a result, when an abdominal mass is observed, surgeons should carefully investigate the patient's past surgical history while taking the possibility of gossypiboma into consideration.