Encephalitis associated with anti-N-Methyl-D-aspartate (NMDA) receptor is a rare form of autoimmune encephalitis. We report the first case of anti-NMDAR encephalitis in an unmarried 16-years old female who was admitted to the Neurology Emergency Unit Faculty of Medicine, Udayana University, Sanglah General Hospital Bali, Indonesia due to decreased consciousness, repetitive talking, headache, involuntary movements in the mouth and feet, and seizures. She was initially diagnosed with viral encephalitis and symptomatic epilepsy. After four weeks of treatment, she was referred to the Gynecology Department. Rectal ultrasound revealed a cystic lesion with a solid component measuring 3.6x2.64x3.18 cm from the left ovary. Laparotomy cystectomy was performed, and the histopathological examination revealed glial cells and mesoderm components in the form of cartilage tissue. Serum and cerebrospinal fluid were positive for anti-NMDA receptor antibodies. She was treated with human intravenous immunoglobulin and rituximab. Her condition was improved gradually. She recovered fully after almost six weeks of hospitalisation.