An eight-year-old boy presented with a fever, abdominal pain, and vomiting. Imaging studies revealed a 10-cm cystic lesion in his right retroperitoneal area with no blood flow and with a pedunculated nodule inside the cavity. He required surgery to remove the lesion, when severe adhesion of the lesion to duodenum and a small vessel-like connection between the lesion and inferior vena cava were dissected to remove the lesion. A histopathological examination revealed that the wall of the main lesion and the vessel-like connection were composed of smooth muscle, elastic and collagen fiber, and stained positive with CD31, which led to the diagnosis of venous malformation. The pedunculated nodule was a fibrin clot with calcification. His postoperative course was uneventful.Venous malformation is a common vascular malformation, but that in the retroperitoneal area has rarely been reported in adults and never in children. Imaging studies often hardly make the correct diagnosis, and then surgery and pathological examination of the resected specimen deem necessary. Careful surgical maneuvers are mandatory due to the possibility of dense adhesions with the surrounding vital organs and tissue. Keywords: Venous malformation, Retroperitoneal, Child