Fetus in fetu (FIF) is a rare entity, with a difficult preoperative diagnosis, frequently being an intraoperative surprise. In recent literature, theories pertaining to its development either assimilate the tumor with a monochorionic twin that halted its evolution, or with a highly differentiated mature teratoma. We present the case of a 27-year-old patient, with a cystic tumor in the anterior mediastinum, which intraoperatively proved to be a rare case of intrathoracic FIF. The clinical presentation lacked any specific clues that would infer such a diagnosis; imaging was partially useful as it shown the possible existence of bone structures, heterogeneously mixed with fatty inclusions and other types of tissue. Upon surgery, the nature of said tumor was clear, and pathology confirmed the FIF diagnosis, showing different types of epithelia and tissue of several organ-like structures that halted in evolution at an early stage. Concluding our presentation, we can say that the presence of an axial skeleton as well as differentiated tissue types of several organs could confirm our case of FIF.