Autoimmune encephalitides (AE) comprise a group of autoimmune diseases of the nervous system that primarily affect the brain’s gray matter. The pathogenesis of AE is largely driven by the production of autoantibodies targeting intracellular and extracellular structures within the nervous system. The most extensively studied forms are those associated with antibodies targeting the NMDAR, AMPAR-, GABAβR-, LGI1, and Caspr2 extracellular antigens. Evidence suggests that anti-NMDA receptor encephalitis occurs more frequently than viral encephalitis in individuals under 30 years of age. Anti-NMDA receptor encephalitis is typically considered in young patients presenting with a primary psychotic episode, catatonia, or epileptic syndrome. However, literature indicates that anti-NMDA receptor encephalitis may present with only a single group of clinical manifestations, rather than the full spectrum of symptoms outlined in the diagnostic criteria, which include abnormal behavior, mental disorders or cognitive dysfunction, impaired consciousness, speech disorders, epileptic syndrome, motor disorders, and autonomic dysfunction. Such monosymptomatic variants can be challenging to diagnose due to mild, nonspecific symptoms and the limited diagnostic value of paraclinical data. We present a case of a patient with rapidly progressive cognitive and behavioral disorders, with no other clinical symptoms, who was diagnosed with anti-NMDA receptor encephalitis and was suspected of having a high titer of anti-NMDA receptor antibodies in both blood and cerebrospinal fluid. A good clinical response to immunosuppressive therapy, along with a recurrent course and repeated detection of autoantibodies in the cerebrospinal fluid, confirmed the primary assumption that the patient had a rare monosymptomatic variant of anti-NMDA receptor encephalitis.