Gossypiboma, a retained surgical sponge, is a rare complication that can occur after any type of surgery. Despite the precautions, the retention of foreign bodies still occurs. We describe a case of a 33-year-old woman with epigastric pain. She was initially diagnosed with acute cholangitis with choledocholithiasis. Although common bile duct stone was successfully removed endoscopically, her epigastric pain did not completely subside. She had undergone an emergency caesarean section at a suburban maternity hospital 6 weeks prior to the referral. A contrast-enhanced CT revealed an encapsulated mass showing a spongiform pattern with fluids and gas bubbles inside, and gossypiboma was suspected. A retained surgical sponge without radiopaque markers was removed surgically. Except for a wound infection, the postoperative course was uneventful. Gossypiboma should always be considered in the differential diagnosis of indeterminate abdominal pain, infection or a mass in patients with a prior surgical history.