Fetus in fetu is a rare congenital anomaly, when the malformed monozygotic twin is found within the body of a living child or sometimes in an adult. We report a case of fetus in fetu diagnosed during the early third trimester of pregnancy, in the medial and inferior portions of anterior mediastinum of the dominant twin. The diagnosis was suspected by prenatal ultrasound and fetal magnetic resonance imaging. It was postnatally confirmed by CT and RTG, surgically removed after birth and histologically examined. Histologically we found well-developed organs and tissues derived from all germ layers. To our knowledge, we describe the fourth case of fetus in fetu located in the thorax, and the second case located in the anterior mediastinum. This case highlights the usefulness of prenatal ultrasound and MRI in the diagnosis of mediastinal masses, and the usefulness of complete histological examination for confirmation of diagnosis of fetus in fetu. We also reviewed the possible embryologic backgrounds of this congenital anomaly.