We report the case of a 65-year-old man with COVID-19 (coronavirus disease-2019) post-infectious encephalitis who presented with delirium as an initial manifestation. He had severe COVID-19 pneumonia and recovered with dexamethasone and tocilizumab. One week after discharge, he developed abnormal behavior and delirium without fever and respiratory symptoms. Brain magnetic resonance imaging showed no abnormalities. Cerebrospinal fluid showed pleocytosis and elevated protein concentrations and was negative for severe acute respiratory syndrome-coronavirus-2 RNA. No anti-neuronal autoantibodies against intracellular and neuronal surface proteins were detected. The cerebrospinal fluid inflammatory changes compatible with post-infectious encephalitis, and the patient recovered with intravenous methylprednisolone and intravenous immunoglobulin therapy. Delirium could be an initial symptom of post-infectious encephalitis in older adults with COVID-19, and these patients may require immunosuppressive therapy.