Avelumab is an anti-PD-L1 (programmed death-ligand 1) immune checkpoint inhibitor (ICIs) and the monoclonal antibody that constitutes a major development in the immunotherapy of cancer. In 2017, The European Medicine Agency (EMA) approved it as an orphan drug for treatment of gastric cancer. Avelumab has recently been approved in the United States, Europe and Japan for treatment of metastatic Merkel cell carcinoma (MCC). Avelumab inhibits the interaction of Programmed cell death protein 1 (PD-1) on immune cells with PD-L1 on tumor cells, thus banishing immunosuppressive signals and leading to enhanced immune cell activation. Here we are revealing a case of the patient with metastatic gastric cancer receiving avelumab with the development of undesirable endocrinopathies during the course of treatment. We suggested that patients receiving avelumab immunotherapy should be monitored for signs and symptoms of thyroiditis, hypothyroidism and adrenal insufficiency, which may require immediate attention and supportive treatment by immunosuppression and respective hormone replacement.