A 72-year-old man with extensive-stage small-cell lung cancer was treated with carboplatin, etoposide, and durvalumab as first-line therapy. After one course of chemotherapy, dysarthria and discomfort in the left upper limb appeared. Following a detailed examination, the patient was diagnosed with autoimmune encephalitis caused by durvalumab. Discontinuation of durvalumab and administration of steroids and intravenous gamma globulin improved the patient’s symptoms rapidly. Although autoimmune encephalitis is an uncommon cause of immune-related adverse events, it should be treated promptly because a delay in diagnosis can lead to serious complications.