Combination treatment with an anti-programmed cell death-1 (PD-1) antibody, an immune checkpoint inhibitor (ICI), and chemotherapy is the standard treatment for patients with HER2-negative advanced gastric/esophagogastric cancer (AGC). ICI re-administration has been reported to have a clinical benefit for patients with lung cancer or melanoma. However, data on patients with AGC have not yet been collected. We plan to conduct a prospective, multicenter, observational NIVO RETURNS study to evaluate the efficacy and safety of nivolumab monotherapy re-administration in patients with AGC refractory to initial anti-PD-1 or anti-programmed cell death ligand-1 (PD-L1) antibody treatment. Patients who have achieved clinical benefits (complete response, partial response, or stable disease for ≥ 6 months) from prior treatment, including anti-PD-1/PD-L1 therapy, will be included. The primary endpoint will be the objective response rate to nivolumab re-administration. We anticipate that our findings will contribute to the improvement of survival outcomes as later-line treatment for AGC. UMIN000050515, UMIN000051044.