The 8th edition of the American Joint Committee on Cancer (AJCC) manual divides T1 stage testicular cancer into T1a and T1b, but it is only applicable to seminoma. The purpose of this observational study is to discuss further the possibility of extending this classification system to any T1 testicular cancer.Testicular cancer patients from 2000 to 2018 in the Surveillance, Epidemiology, and End Results (SEER) database were included in this analysis. After patient selection, univariate and multivariate Cox regression were used to evaluate the impact of tumor size on survival in patients with T1 testicular cancer. A time-dependent receiver operation curve (ROC) was used to determine the best tumor size cut-off value for further T1 subgroup classification. Restricted cubic splines (RCS) analysis was used to compare different tumor sizes with the best tumor size cut-off value. Propensity score matching (PSM) analysis was conducted to generate baseline balanced data to validate findings.A total of 6,630 patients were included in this study. In the Cox regression model, we found that T1b staged tumor (>34 mm) was an independent risk factor of overall survival [OS, adjusted hazard ratio (HR): 1.57, 95% confidence interval (CI): 1.12-2.21] and cancer-specific survival (CSS, adjusted HR: 5.027, 95% CI: 1.95-12.93). Further PSM analysis consolidated our results.For any T1 testicular cancer, a tumor size of 34 mm could be used as the demarcation point to assess the prognosis. Adopting personalized treatments and follow-up plans may help improve the OS and CSS rate for testicular cancer patients.2024 Translational Andrology and Urology. All rights reserved.