Mature cystic teratomas, or dermoid cysts, are part of the family of non–seminomatous germ cell tumors composed of somatic cells from at least two germ layers. They usually present as a pelvic mass arising from the gonads. The presence of teratomas in the retroperitoneum is a rare phenomenon occurring due to sequestration of germ cells from the yolk sac during migration. We present a case of an abdominal lump in a 20 year–old female which was provisionally diagnosed as a teratoma on imaging. A formal exploration was done and the lump, weighing approximately 7kg, was excised. Lump was confirmed as a mature teratoma of subtype Fetus in Fetu on histopathology. This rare congenital anomaly has been defined as the presence of a malformed and parasitic fetus in the body of its twin. Its incidence is about 1:500,000 live births. Retroperitoneal teratoma is rare and even rarer is the possibility of a Fetus in Fetu. This case is being reported with relevant salient features and is discussed in the light of available literature due to its distinctive characteristics