Background: Expression of the PD-1 ligands PD-L1 and PD-L2 has been reported in ≈ 40% of pts with E/GEJ cancer and is associated with a poor prognosis. In a phase 3 trial, the PD-1 inhibitor nivo demonstrated an OS benefit vs placebo (HR, 0.63; P< 0.0001), resulting in a 37% reduction in the risk of death and double the OS rate at 12 mo (27% vs 11%) in pts with advanced gastric (G)/GEJ cancer refractory to ≥ 2 lines of chemotherapy (Kang YK, et al. J Clin Oncol. 2017;35 (suppl 4S) [abstract 2]). In this study, nivo was well tolerated, with a safety profile comparable with that of the placebo arm. These results indicate that nivo could be a new standard of care (SOC) for pts with heavily pretreated advanced G/GEJ cancer and provide a strong rationale to explore nivo in earlier lines of treatment for G/E/GEJ cancer. Currently, no effective adjuvant SOC is available after chemoradiotherapy (CRT) followed by resection for pts with E/GEJ cancer. This multinational, double-blind, phase 3 trial will evaluate nivo as an adjuvant therapy for pts with resected E/GEJ cancer (CheckMate 577; NCT02743494). Methods: In this study, an estimated 760 pts aged ≥ 18 years with stage II/III E/GEJ cancer are randomized to receive nivo or placebo. Prior to randomization, pts must have completed preoperative CRT followed by surgery and been diagnosed with residual pathologic disease after being surgically rendered free of disease with negative margins following complete resection. Pts with stage 4 resectable disease, cervical esophageal cancer, or those who have not received concurrent CRT prior to surgery are not eligible for study enrollment. Primary endpoints are OS and disease-free survival. Other key endpoints include the OS rate at 1, 2, and 3 years and safety. Clinical trial information: NCT02743494