Background: Although a number of studies have investigated whether tumor diameter is a prognostic factor in gastric cancer, no consensus was reached on its clinical importance. In this study, we aimed to determine the effect of tumor size on survival in patients with pT3 gastric cancer. Patients and Methods: A total of 232 patients with pT3 gastric cancer, who underwent curative gastrectomy with D2 lymph node dissection, were retrospectively analyzed. Receiver operating characteristics analysis showed that the cutoff value for tumor size was 8 cm. On the basis of this cutoff point, patients were divided into 2 groups: small-size tumors (SST, <= 8 cm) and large-size tumors (LST, >8 cm). The prognostic significance of tumor size and the relationship between tumor size and other prognostic factors were evaluated. Results: LST was detected in 44% of patients. Resection type, tumor site, lymph node metastasis, tumor differentiation, lymphatic vessel invasion, and blood vessel invasion were correlated with tumor size. The median survival of patients with SST was significantly better than that of patients with LST (107 vs. 18.2 months; p < 0.001). Multivariate analysis indicated that tumor size was an independent prognostic factor (p = 0.001; hazard ratio (HR): 0.43) as were resection type and blood vessel invasion. Conclusions: Our results show that tumor size is an important prognostic indicator in patients with pT3 gastric cancer, who underwent curative gastrectomy, and that the rate of LST increased with aggressiveness and stage of disease. Tumor size may be a useful and reliable prognostic factor for detection and staging in patients with gastric cancer, who have a poor prognosis after curative resection.