The fifth International Union Against Cancer tumor node metastasis (UICC TNM) classification, based on the number of metastatic lymph nodes (LN), has proved to be a reliable and objective method for predicting the prognosis of patients with gastric cancer. However, the prognosis of patients with T3 gastric cancer is still heterogeneous. This study was carried out to investigate the validity of metastatic LN ratio as a prognostic factor in T3 gastric cancer. A retrospective analysis was performed on a total of 833 patients that had either T3N1M0 (n = 504) or T3N2M0 (n = 329) gastric cancer by the fifth UICC classification. A preliminary analysis revealed the cutoff values for T3N1M0 to be 10% and for T3N2M0 to be 25%. The mean metastatic LN ratio was 9.0% for T3N1M0 cancer and 26.9% for T3N2M0 cancer. For the T3N1M0 stage, the patients who showed less than 10% of the metastatic LN ratio were grouped as N1-low with the others grouped as N1-high. For the T3N2M0 stage group, those who had less than 25% of the metastatic LN ratio were grouped as N2-low, the remainder as N2-high. The metastatic LN ratio decreased in proportion to the extent of lymphadenectomy and it increased in relation to the increasing scale of the fourth N classification. The rates of recurrence were significantly different according to the metastatic LN ratio in N1 and N2 classification of the fifth UICC classification (p < 0.05). The 5-year survival rates after gastrectomy decreased significantly by increasing the metastatic LN ratio in both T3N1M0 cancers (p =0.0026) and T3N2M0 cancers (p = 0.0057). The metastatic LN ratio was an independent risk factor for recurrence and poor prognosis. Our data suggest that the metastatic LN ratio is a significant prognostic factor for T3 gastric cancer. Furthermore, the application of the metastatic LN ratio can provide information not only about the extent of LN metastasis but also about the extent of lymphadenectomy in T3 gastric cancer.