Anti–N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis is an acute autoimmune neurological disorder that presents with acute to subacute psychiatric and/or neurological complaints including new onset behavioral changes that may evolve to psychosis and catatonia, cognitive decline, new onset seizures, progressive encephalopathy, and/or movement disorders. Female teens and adults often have an associated ovarian teratoma as an underlying etiology, but most pediatric patients do not have an identifiable associated neoplasm. The diagnosis requires confirmatory serum and/or cerebrospinal fluid analysis findings of anti-NMDAR antibody titers. It can be misdiagnosed as a psychiatric condition or a viral encephalitis. The clinical features that distinguish anti-NMDAR encephalitis from a primary psychiatric disorder are the acute onset of the mood and behavioral changes with no history, the waxing and waning of consciousness (delirium/encephalopathy), and primary neurological features such as seizures and abnormal involuntary movements, including dyskinesias and dystonias. The prognosis is improved with earlier recognition and prompt immunotherapy treatment, making this an important diagnosis for emergency physicians.