<p>Immune checkpoint inhibitors, such as nivolumab, have become pivotal in the treatment of advanced gastric cancer, particularly when combined with chemotherapy. While immune-related adverse events (irAEs) are commonly viewed as complications, their occurrence has been increasingly associated with favorable treatment responses. We report the case of a 59-year-old man diagnosed with advanced gastric cancer with liver and multiple lymph node metastases. The patient received chemotherapy, including nivolumab, but developed multiple grade 3 irAEs during treatment, including hypophysitis with secondary adrenal insufficiency, type 1 diabetes mellitus, and late-onset colitis. Despite these toxicities, imaging and endoscopy showed complete disappearance of the primary tumor and metastatic lesions, and a complete response was achieved per the Response Evaluation Criteria in Solid Tumors. Nivolumab monotherapy was continued for maintenance until gastrointestinal symptoms recurred due to irAE-associated colitis, which was successfully treated with corticosteroids. No tumor relapse was observed during this period. This case highlights that multi-organ irAEs are predictive of a favorable treatment response and that appropriate treatment of irAEs may result in excellent treatment outcomes in advanced gastric cancer.</p>