Adjuvant chemotherapy with S-1 is a standard treatment for patients with stage II/III gastric cancer after D2 gastrectomy; however, this is not uniformly applicable in older patients. The Stomach Cancer Study Group of the Japan Clinical Oncology Group conducted a questionnaire survey on older patients aged ≥80 years and revealed wide heterogeneity among older patients of similar chronological age. This randomized trial aims to confirm the superiority of modified S-1 treatment (reduced initial dose of S-1 by ≥ 1 level compared with the standard dose) versus surgery alone in vulnerable older (≥80 years old) patients with pathological stage II/III gastric cancer after curative resection. A total of 370 patients will be enrolled from 62 institutions over 4.5 years. The primary endpoint of this study is overall survival. This trial has been registered in the UMIN Clinical Trials Registry with code UMIN000025742 [http://www.umin.ac.jp/ctr/index.htm].