Fetus in fetu is an extremely rare condition in which a fetus or fetus-like structure with a vertebral axis is seen in the body of its twin. This paper presents a case of fetus in fetu in a twoday-old female newborn who was referred for an abdominal mass, biliary vomiting, and feeding intolerance. After plain abdominal X-ray and ultrasonography, the patient underwent abdominal surgery with the primary diagnosis of teratoma or fetus in fetu. We found a retroperitoneal mass that consisted of double fetus in fetu and a separate undetermined mass. The pathologic examination confirmed double fetus in fetu and revealed a separate immature teratoma. She was discharged from the hospital after seven days in a healthy and normal condition. The level of serum alpha-fetoprotein was normal after three months of follow-up.