Background: Tumor size is still considered a useful prognostic factor in currently available tumor-node- metastasis (TNM) classification staging systems for most solid tumors, but the significance of tumor size on the prognosis of ampullary carcinoma remains controversial. The aim of the current study was to propose a new T-stage classification system for ampullary carcinoma to address the impact of tumor size on the prognostic outcome.Methods: Using the Surveillance, Epidemiology, and End Results (SEER) database, we identified 1080 patients with ampullary carcinoma who underwent radical surgical resection between 2004 and 2015. Based on the results obtained from analysis of various clinicopathologic factors, a new T-stage classifica- tion system was proposed. Results: Among the 1080 patients, 618 were men and 462 were women, with a median tumor size of 2.3 (range 0.1–12) cm. Using the 7th edition of the American Joint Committee on Cancer (AJCC) staging manual, we noticed significant differences in overall survival (OS) between T2 vs. T3 tumors ( P < 0.001) and T3 vs. T4 tumors ( P = 0.002), but failed to observe significant differences between T1 vs. T2 tumors ( P = 0.498) in our pair-wise comparison. Using the newly developed T-stage classification system, we were able to differentiate significant differences in OS between T1 vs. T2 tumors ( P = 0.032), T2 vs. T3 tumors ( P < 0.001) and T3 vs. T4 tumor ( P = 0.003) in all pair-wise comparisons. The c-index of the new staging system was 0.653 (95％ CI: 0.629–0.677), showing a better discriminatory power than the 0.636 of the 7th AJCC staging system (95％ CI: 0.612–0.660). Conclusions: The new T-stage classification system described herein can better differentiate prognostic outcomes after radical resection in patients with ampullary carcinoma by incorporating tumor size and depth of tumor infiltration.