Background: Little knowledge is available for postsurgical changes in cognitive and physical functions that may be useful for considering indication for surgery in elderly patients with gastric cancer. We therefore conducted a prospective study aimed to determine the influence of gastrectomy on these patients. Methods: We recruited patients older than 75 years for whom gastrectomy for gastric cancer had been planned, and assessed their cognitive and physical functions, daily activities, episodes of depression, confusion, and delirium before surgery (baseline), upon discharge, and at 6 months after surgery (POM 6). Results: Among 54 elderly patients registered between February 2017 and February 2020. There were no significant decreases in MMSE scores between baseline and at POM 6, nor were there significant differences in physical function and indicators of depression and confusion between these time points. As many as 20% of patients were found to have the functional decline on the basic activities of daily living scores (BADL) after surgery compared with the baseline. The only variable significantly associated with a functional decline in BADL was postoperative complications. Conclusions: Postoperative cognitive functions did not significantly decline when compared with the baseline scores, although postoperative BADL scores of patients who experienced postoperative complications were significantly lower than those who did not.