Stage III gastric cancer is characterized by locally advanced disease with varying anatomic extent as measured by the 7 edition of the American Joint Committee on Cancer (AJCC) staging system. There are no prognostic factors specifically identified in patients with stage III gastric cancer following extended lymph node dissection (D2) surgery. From 2007 to 2014, 534 patients with stage III gastric cancer underwent radical gastrectomy and D2 dissection at the Chang Gung Memorial Hospital. Patients' characteristics and the impact of adjuvant chemotherapy were analyzed using univariate and multivariate analyses to identify variables associated with overall survival (OS) and disease-free survival (DFS). There were 320 deaths (60.0%) and 284 recurrences (53.2%) by the end of the study. The median OS and DFS were 30.7 months (95% confidence interval [CI]: 27.5-33.9) and 26.4 months (95% CI: 21.2-31.6), respectively. The multivariate analysis identified 7 variables that were independent prognostic factors both for OS and DFS including ratio of metastatic lymph nodes to total resection lymph nodes, carcinoembryonic antigen level, Eastern Cooperative Oncology Group performance status, gastrectomy method, vascular invasion, surgical margin, and adjuvant chemotherapy. Patients with stage IIIA-IIIC disease who received adjuvant chemotherapy had better OS and DFS outcomes than those who did not. Our study identified several independent prognostic factors that might help determine the appropriate counseling patients following surgical treatment. D2 surgery alone was inadequate to achieve long-term survival. As the only correctable independent prognostic factor, postoperative adjuvant chemotherapy should be recommended for eligible patients with stage III gastric cancer.