Aberrant β1, 6-N-acetylglucosaminyltransferase V (MGAT5) expression in malignant tissues has been reported to be involved in the development of various cancers and their progression, through altering N-glycan branching. We aimed to investigate the clinical and prognostic values of MGAT5 and improve the risk stratification in patients with gastric cancer. MGAT5 expression was retrospectively analyzed by immunohistochemistry in three independent sets comprising 313 patients from China with gastric adenocarcinoma. Results were assessed for association with clinical features and overall survival using Kaplan–Meier analysis. Prognostic values of MGAT5 expression and clinical outcomes were evaluated by Cox regression analysis. A molecular prognostic stratification scheme incorporating MGAT5 expression was determined in patients with late-stage gastric cancer by using receiver operating characteristic analysis. The results show that low intratumoral MGAT5 density, which was associated with poor differentiation, N classification, TNM stage, and Kiel stage, was an independent prognosticator for poor overall survival. The combination of intratumoral MGAT5 expression and TNM or Kiel staging systems had a better predictive power for overall survival. Applying the prognostic value of intratumoral MGAT5 density to TNM stage III+IV and Kiel stage IIIB+IV groups showed a better risk stratification for overall survival in patients with late-stage gastric cancer. In conclusion, integrating intratumoral MGAT5 density that was recognized as an independent prognostic marker into current clinical staging systems significantly improved prognostic stratification of patients with late-stage gastric cancer. This refined risk stratification scheme might aid in appropriate therapeutic options and ultimately improve the outcomes of patients with advanced-stage disease.