PURPOSE: To evaluate the effect of adjuvant chemotherapy on improving the prognosis of patients with stage I triple-negative breast cancer (TNBC). METHODS: TNBC patients diagnosed in the SEER 18 database from 2010 to 2015 were included. Kaplan-Meier plots and log-rank tests were used to compare the differences in breast cancer-specific survival (BCSS) and overall survival (OS) between subgroups of variables. A Cox proportional hazard model was used to determine the prognostic factors affecting BCSS and OS. RESULTS: A total of 9256 patients were enrolled in this study. Among these patients, 380 died from breast cancer, and 703 died from all causes. Patients who received chemotherapy had significantly better BCSS and OS than those who did not receive chemotherapy for stage T1cN0M0 (BCSS, hazard ratio (HR) = 0.68, 95% confidence interval (CI) 0.51-0.90; OS, HR = 0.54, 95% CI 0.44-0.67) and stage IB (BCSS, HR = 0.39, 95% CI 0.16-0.95; OS, HR = 0.41, 95% CI 0.19-0.87) disease. Patients who received chemotherapy did not have significantly better BCSS or OS than those who did not receive chemotherapy for stage T1aN0M0 or T1bN0M0 disease. The patients who received chemotherapy in the poorly differentiated and undifferentiated groups had better BCSS (HR = 0.68, 95% CI 0.52-0.88) and OS (HR = 0.54, 95% CI 0.44-0.66) than the patients who did not receive chemotherapy. CONCLUSION: According to current clinical guidelines, patients with stage T1bN0M0 TNBC are probably overtreated. The prognosis of these patients with stage T1aN0M0 or T1bN0M0 disease is good enough that adjuvant chemotherapy cannot improve it further.