In the current practice of oncology, immunological treatments have an expanded spectrum owing to increased efficacy in various indications for the same tumor and the rising number of responsive cancers, by the novel agent development. In this report, the emergence of a peritoneal lesion is presented in a 50-year-old male patient under nivolumab treatment for lung adenocarcinoma. Granulomatous tissue with dense histiocytic infiltration and necrotic foci was revealed on histopathological examination. Immune-related granulomatous lesions are commonly defined as sarcoid-like and no necrosis, generally located in the mediastinal lymph nodes, lungs, and skin. To the best of our knowledge, this is the first report of an immune-related granulomatous lesion with histiocytic infiltration and necrotic foci, located on the peritoneum. The etiopathogenesis and clinicopathological features of immunotherapy-related granulomatous lesions and their potential predictive role on anticancer efficacy should be better understood.