Purpose <p>The present study aims to investigate the prognostic impact of low physical performance in elderly patients undergoing radical gastrectomy for gastric cancer, and to explore the risk factors for low physical performance in these patients.</p> Methods <p>Clinical data of 840 elderly patients (≥ 65 years) who underwent radical gastrectomy for gastric cancer were prospectively collected. Physical performance was assessed by 6-m usual gait speed test. Muscle strength was measured by grip strength test and chair rise test. Body composition was analyzed by preoperative computed tomography (CT).</p> Results <p>Low physical performance was associated with higher incidence of ileus and severe complications after surgery. Low physical performance was independently associated with worse overall survival and disease-free survival after adjusting for age, gender, tumor–node–metastasis (TNM) stage, and histological differentiation of tumor. Physical performance had a significant correlation with skeletal muscle index (SMI), skeletal muscle density (SMD), chair rise time, and grip strength, but not with fat mass. Females, nutritional risk, chair rise time ≥ 15 s, low grip strength, low SMD, and higher TNM stage of tumor were independent risk factors for low physical performance in elderly patients with gastric cancer.</p> Conclusion <p>Low physical performance was associated with worse postoperative outcomes in elderly patients undergoing radical gastrectomy for gastric cancer. Aging itself was not independently associated with low physical performance. Nutritional support, exercise strategies aiming to improve muscle strength and density, and anti-tumor treatments to reduce tumor burden could be beneficial to improve physical performance, which could further improve the prognosis in elderly patients with gastric cancer.</p>