A 21-year-old woman was admitted with the chief complaint of fullness of the upper left abdomen. Her past and family medical history was unremarkable. Abdominal examination revealed a spontaneous swelling and tenderness over the ribs on the left side. There were no signs of peritoneal irritation. Laboratory examination at admission showed no abnormalities, except for elevation of the serum level of the tumor marker CA19-9 to 163 U/ml. Abdominal contrast-enhanced CT showed a cystic mass measuring 170×160×100 mm in size in the upper left abdomen, and the contrast enhancement of the cyst margin suggested a splenic origin of the lesion. The imaging findings showed no apparent evidence of infection. A giant splenic cyst was diagnosed and laparoscopic splenectomy was performed. The operation time was 3 hours and 30 minutes, and the intraoperative blood loss was 50 ml. The postoperative course was uneventful and the patient was discharged 10 days after the operation. Histopathological examination showed epithelial cysts and no evidence of malignancy. The serum CA19-9 levels returned to within normal range immediately after the surgery.