Anti-N-Mmethyl-D-aspartate receptor encephalitis is an autoimmune disease of the central nervous system with prominent neurologic and psychiatric features. Symptoms appear progressively and sometimes with an exclusively psychiatric initial presentation. The patient’s evaluation should be meticulous, and we should use all the diagnostic tests required for the exclusion of entities that can mimic this disease. We report the diagnostic investigation of a case of anti-N-methyl-D-aspartate receptor encephalitis in a patient with a previous diagnosis of schizophrenia with poor response to antipsychotics. The aim of this case report is to highlight the importance of close surveillance for neuropsychiatric symptoms, especially catatonia, and to recognize autoimmune encephalitis in the differential diagnosis of psychotic disorders with neurological symptoms and resistance or intolerance to antipsychotics. A prompt diagnosis will contribute to a faster onset of therapy and an overall improvement in prognosis.