Background: Triple-negative breast cancer (TNBC) has been associated with poorer long-term outcomes compared to other breast cancer subtypes. Due to the lack of targeted therapies, the use of adjuvant chemotherapy is the mainstay of treatment, and its efficacy has demonstrated to be correlated with tumor size. Currently, different national guidelines exist regarding chemotherapy in early stage I TNBC. This study aims to examine the outcomes of the use of adjuvant chemotherapy in stage I TNBC stratified by tumor size and treated in Ontario, using the ICES database. Methods: Records of stage I TNBC patients diagnosed in 2012 to 2014 were collected from the ICES database and analysed. Kaplan-Meier curves and log-rank tests were used to compared differences in overall survival (OS) between groups. Stage I TNBC cancer patients were analyzed by tumor size: T1a (< 0.5 cm), T1b (> 0.5 cm and < 1.0 cm) and T1c (>1.0 cm and < 2.0 cm). A Cox proportional hazard model was used to determine significance of variables. Results: Of the 610 stage I TNBC patients, 183 had tumor sizes ≤ 1cm, representing stages T1aN0M0 and T1bN0M0, and 427 had tumor sizes > 1cm to 2cm, representing stage T1cN0M0. Patients with tumor sizes ≤ 1cm who received adjuvant chemotherapy did not have a significant difference in OS compared to those who did not receive adjuvant chemotherapy (p = 0.41, hazard ratio (HR) = 0.40, 95% confidence interval (CI) 0.021-2.5). However, patients with tumor sizes > 1cm to 2cm who received adjuvant chemotherapy demonstrated a significantly better OS compared to those without (p = 0.023, HR = 0.40, 95% CI 0.16-0.86). Conclusions: Patients with TNBC stage T1cN0M0 should receive adjuvant chemotherapy for better OS. For stages T1aN0M0 and T1bN0M0, patients may be overtreated and avoidance of adjuvant chemotherapy can be considered at this time. Prospective studies should investigate the efficacy of adjuvant chemotherapy in TNBC patients with stage T1aN0M0 and T1bN0M0.