Background: Mature teratoma is a benign neoplasm, mostly composed of well-differentiated derivations of almost two or three germ cell layers, while immature teratoma is a malignant neoplasm composed of immature neural and embryonic tissue. Immature teratoma in the context of ovarian endometrioma has not been reported yet. Case Presentation: A 34-year-old woman with primary infertility is reported in this study who suffered from immature teratoma associated with ovarian endometrioma. After several rounds of fertility treatment, the patient was referred for severe abdominal pain and underwent emergency surgery for the rupture of ovarian endometrioma. To preserve the ovary, the cyst was not resected in areas attached to the ovary. Some months later, the patient noticed a progressive abdominal enlargement. The sonographic evaluation revealed multiple solid-cystic lobulated masses on the abdominal wall and throughout the pelvic cavity. The histologic diagnosis was consistent with immature teratoma. The patient was treated with high-dose neoadjuvant chemotherapy and fertility-sparing surgery (FSS). The histologic evaluation of the extracted masses revealed teratoma maturation. Conclusion: This study reveals the importance of complete removal of endometrioma and highlights the role of neoadjuvant chemotherapy in fertility-sparing surgery and potentiating teratoma maturation.