Recently, immune checkpoint inhibitors (ICIs) have become the standard treatment option for patients with lung cancer, including small cell lung cancer (SCLC). ICI-induced neurological immune-related adverse events are rare and exhibit diverse clinical manifestations, often leading to missed or delayed diagnosis. Herein, we report the case of a patient with extensive-stage SCLC who received atezolizumab with etoposide/platinum and gradually developed neurological symptoms after three cycles of chemoimmunotherapy. Subsequently, the patient received a diagnosis of subacute immune-related cerebellar ataxia and was treated successfully with pulse steroid therapy. The patient exhibited almost complete remission of neurological symptoms and had progression-free survival for >24 months.