Among the subtypes of germ cell tumors, teratomas are the most frequent in the pediatric population and commonly occur in the sacrococcygeal region and the gonads. Less than 1% of all teratoma are found in abdominal organs including the stomach, liver, and kidney. Gastric teratomas are very rare tumors predominantly found in infants. Moreover, an immature gastric teratoma is exceptionally rare. Here, we present a case of immature gastric teratoma with spontaneous rupture in a newborn who was preoperatively diagnosed with neuroblastoma. On the first day after birth, the neonate presented with progressive abdominal distension accompanying respiratory distress. A firm mass was detected during a physical examination of the abdomen. An emergency exploratory laparotomy revealed hemoperitoneum resulting from a rupture of the tumor located in the posterior wall of the gastric antrum. Complete resection of the tumor and gastroduodenostomy were performed. The pathology evaluation revealed a grade 3 immature gastric teratoma with no malignant components. The patient was treated with adjuvant chemotherapy to prevent recurrence, since the tumor was ruptured in the abdominal cavity and the level of alpha-fetoprotein was decreased but still remained high above the normal range after surgery. In conclusion, physicians should be aware of the existence of gastric teratoma as the differential diagnosis of a huge abdominal mass in infants, especially neonates. Complete surgical removal of the tumor and long-term follow-up has been adopted as the standard management for immature gastric teratoma, although there has been controversy with adjuvant chemotherapy.