Gliomatosis peritonei (GP), almost exclusively linked to mature or immature ovarian teratoma, is a very rare disease. To the best of our knowledge, reports on the complete clinical course and imaging features of ovarian mature teratoma with GP are extremely rare. We present a case of ovarian mature teratoma with GP in a 9-year-old girl admitted to the emergency department for a 2-month history of a large abdominal mass found accidentally. Carcinoembryonic antigen and cancer antigen 125 levels were elevated. CT scans suggested a large mass with mild enhancement, and an immature teratoma derived from the left ovary with ascites was diagnosed by ultrasound. Subsequently, left ovarian tumor resection and omentectomy were performed, and a solid cystic mass accompanied by massive ascites and numerous white to grayish nodules was identified on the left ovary. The pathology results revealed a mature teratoma with GP. The patient had good postoperative recovery, and her serum tumor marker levels decreased to normal at the 3-month follow-up.