Background <p>Fetus in fetu (FIF) is identified by the appearance of a partially formed fetus inside a twin which is an extremely rare congenital pathology. This rare disease has about 200 reported cases globally. FIF can arise from a deviation in the twinning process. It usually occurs within the abdomen, often in the retroperitoneal region, and typically presents during infancy as an abdominal mass. FIF rarity remains a diagnostic challenge and diverse clinical symptom. The recommended treatment is complete surgical removal of the mass. Early intervention provides a good prognosis.</p> Clinical Description <p>A female infant was taken to the emergency department by her parents due to her enlarged abdomen. Physical examination discovered a cystic mass in the left abdominal cavity. Surgery was performed with an exploration of the abdomen. We found a well-encapsulated mass in the retroperitoneum area containing the left kidney. To preserve vital structures, cautious dissection was performed and removed the mass successfully without rupture. The patient recovered uneventfully after surgery.</p> Conclusion <p>FIF is diagnostically challenging and often requires laparotomy exploration to release the mass. For favorable results, surgical excision is crucial especially to avoid probable complications.</p>