A previously healthy 21-year-old woman, transported to our medical emergency center for excluding organic brain disease, had undergone medical examination 9 days before for trembling in her left hand, which was caused by stress. The patient exhibited fever and strange behaviors, e.g., wandering around, babbling, and making smoking gestures; hence, psychiatric examination was performed. The patient's Glasgow Coma Scale score was 4-3-5, and involuntary movement was observed. Cerebrospinal fluid examination revealed increased cell count; hence, we suspected anti-N-methyl-d-aspartate (NMDA) receptor encephalitis. We conducted an abdominal CT scan, which revealed a neoplastic lesion with calcification in the right ovary. Early steroid pulse therapy was started. On hospital day 25, she tested positive for anti-NMDA receptor antibodies; hence, anti-NMDA receptor encephalitis and concomitant ovarian teratoma was diagnosid. She underwent right adnexectomy; subsequently, immunotherapy was performed. The patient recovered and was discharged on hospital day 105. Anti-NMDA receptor encephalitis is not uncommon; however, this disease must be considered for young encephalitis patients exhibiting psychiatric symptoms. If patients (aged ≤ 30 years) presents with encephalitis of uncertain etiology, psychiatric symptoms, seizures, movement disorders, or psychosis, clinicians should consider anti-NMDA encephalitis as a possible diagnosis. Clinical diagnosis should be waged early to ensure timely treatment.