Anti–N–methyl–D–aspartate receptor (anti–NMDAR) encephalitis is a treatment–responsive encephalitis associated with anti–NMDAR antibodies. Unlike classic paraneoplastic encephalitis, this disorder usually develops in young women with ovarian teratoma who typically present with marked neuropsychiatric symptoms, followed by prolonged respiratory failure, clouding of consciousness, and bizarre dyskinesia. This disorder is often treatable by resection of ovarian tumor and immunotherapy, but, delayed diagnosis results in a worse condition and sometimes fatal outcome. However, some gynecologists are not familiar with this disorder. When physicians encounter a female patient with encephalitis showing marked neuropsychiatric symptoms, search for an ovarian tumor should be promptly initiated. We present a case of anti–NMDAR encephalitis associated with ovarian immature teratoma. The symptoms were dramatically relieved by tumor resection and immunotherapy.