The monocyte-to-lymphocyte ratio (MLR) is an important prognostic determinant of various malignancies. However, the prognostic role of MLR in patients with gastric cancer undergoing gastrectomy remains unclear. Patients with stage I to III gastric cancer who underwent curative-intent gastric resection were enrolled in this study. Cox regression analysis was used to determine the independent variables for overall survival (OS) and disease-free survival (DFS). The established models were validated internally. Inter-model comparisons were performed using the integrated area under the receiver operating characteristic curve and the concordance index. Multivariate Cox regression analysis revealed that age, tumor–node–metastasis (TNM) stage, perineural invasion, serum albumin level, and MLR were prognostic factors for OS and DFS and constituted the full model. The full model was internally validated using calibration curves and decision curve analysis. The integrated area under the curve and concordance index of the full model outperformed those of TNM stage. The full model was a significant determinant of OS and DFS. Additionally, the full model was suggested to outperform TNM stage in predicting patient survival outcomes.