Growing teratoma syndrome (GTS) consists of germ cell tumors that grow following chemotherapy despite complete eradication of the malignant cells. They can metastasize to any site, particularly the retroperitoneum, mediastinum and cervical region. It typically affects young adults and adolescents. Here we describe the youngest case reported in a 4-year-old girl with an ovarian mixed germ cell tumor who underwent an oophorectomy. Her tumor markers normalized by the end of her chemotherapeutic treatment; however, she developed a retroperitoneal mass that was subsequently resected. Histopathology revealed a mature teratoma, consisting of a GTS. We stress the need for early recognition and treatment of GTS to avoid the subsequent morbidity and mortality associated with it. Although GTS has an excellent prognosis when completely resected, it is essential that the patient be regularly followed-up with serum tumor markers and imaging.