Fetus in fetu is a congenital abnormality in which a nonviable, parasitic fetus grows within its twin. It is a rare cause of retroperitoneal abdominal mass in infants and children. The authors report a recent case of a six-month-old girl who presented with unexplained abdominal distention. A sonogram and a magnetic resonance imaging examination showed a multiloculated, complex cystic mass with calcified and soft tissue components. A definitive diagnosis and discrimination from a teratoma was difficult to make because of the absence of a distinctive criterion, the presence of a vertebral column. Pathologic examination showed a complex mass consisting of well-formed bowel and upper respiratory tract segments as well as mature neuroglial tissue, skeletal muscle tissue fibers, and bone tissue that contained bone marrow, supporting the diagnosis of fetus in fetu. Therefore, the nonvisualization of a vertebral column on imaging should not exclude fetus in fetu from the differential diagnosis.