While immune checkpoint inhibitors (ICIs) have revolutionised cancer treatment, they can cause immune-related adverse events, including thyroid dysfunction and rarely, thyroid storm. Here, we present a rare case of thyroid storm occurring during ICI therapy, in which clinical and immunological findings strongly suggested Graves' disease. A man in his 80s with advanced lung cancer was hospitalised for multiple rib fractures. During admission, he developed fever, delirium, atrial fibrillation and marked thyrotoxicosis, leading to a diagnosis of thyroid storm. Given his prior treatment with nivolumab and ipilimumab, destructive thyroiditis was initially suspected, as this is the most common mechanism of ICI-associated thyrotoxicosis. However, marked thyroid-stimulating antibody positivity was more suggestive of Graves' disease than destructive thyroiditis. This case highlights that ICI therapy-associated thyroid storm can arise not only from destructive thyroiditis but also from Graves' disease. Therefore, careful clinical evaluation is essential to determine the underlying pathophysiology and guide optimal management.