Background: In Japan, endoscopic submucosal dissection (ESD) has been widely performed for ESD-adapted gastric cancer, but little is known about the prognostic factors after ESD for gastric cancer in older patients. The psoas muscle index (PMI) is an indicator of sarcopenia calculated from computed tomography images and reportedly related to the prognosis of some diseases. This study aimed to explore factors related to long-term survival after ESD for gastric cancer in patients aged ≥ 80 years. Methods: We retrospectively reviewed 88 patients (63 men, 25 women) with early gastric cancer who underwent ESD at ≥ 80 years. Possible factors related to death after gastric ESD were analyzed by univariate and multivariate analyses using a Cox proportional hazards model. The estimated overall survival (OS) was compared between the groups stratified by significant factors. Results: The 5-year OS rate was 73.9% (median follow-up period, 5.4 years). In the multivariate analysis, a low PMI (< 6.36 in men, < 3.92 in women) (hazard ratio [HR] 2.89, 95% confidence interval [CI] 1.11–7.54) and high Charlson comorbidity index (CCI) (≥ 3) (HR 1.87, 95% CI 1.14–3.09) were independently related to death after ESD. The 5-year OS rates were significantly higher in the high PMI group (82.3% vs. 70.7%, P = 0.026) and the low CCI group (76.0% vs. 37.9%, P = 0.002). Conclusion: In addition to low CCI, high PMI is a predictor of long-term survival after ESD for gastric cancer in patients aged ≥ 80 years.