Anti-NMDAR encephalitis is a serious central nervous system disorder, not well known, that may lead to non specific psychiatric symptoms, ranging from psychosis to catatonia. We report the case of a 15-year-old girl, without significant medical history, hospitalized in a context of first-episode psychosis, quickly followed by agitated and stuporous catatonia, with only poor response to benzodiazepine treatment. Titration of autoantibodies in the cerebrospinal fluid confirmed the diagnosis of anti-NMDAR encephalitis. Recovery was obtained thanks to corticotherapy and immunosuppressive agents. In complement to this clinical report, we propose a literature update on anti-NMDA-R encephalitis and the excitatory/inhibitory imbalance as the potential mechanism at the root of catatonia.