Autoimmune encephalitis (AE) is a group of inflammatory brain diseases that are characterized by prominent neuropsychiatric symptoms. Early therapeutic intervention is important for AE. Therefore, without waiting for autoantibody test results, clinicians must consider the possibility of AE based solely on clinical symptoms and conventional test results. The case described herein is of antibody-negative encephalitis with abnormalities shown only by EEG, which contributed to the diagnosis and treatment. The patient, a 20-year-old woman, showed autonomic seizures in addition to movement disorders, psychiatric symptoms, and cognitive dysfunction, which worsened subacutely. Her seizures and movement disorders were not responsive to antiepileptic medications. Results obtained from MRI and cerebrospinal fluid (CSF) were normal; EEG findings showed repeated spikes in the right temporal area, with changes over time. Based on the clinical course and EEG, along with administered immunotherapy, which resolved seizures, movement disorders, and psychiatric symptoms, we suspected AE. For diagnosis of AE and for evaluating treatment responsiveness, EEG was useful. Results indicate that EEG can assist clinicians even with AE cases for which MRI and CSF findings are normal.