Background: Outcomes for patients with stage IA TNBC in the modern era remain poorly characterized, and the benefit and utilization of adjuvant chemotherapy (chemo) in this setting remains controversial. Methods: We analyzed data from women diagnosed with pathological stage IA TNBC in the Surveillance, Epidemiology, and End Results (SEER) database during 2010 to 2019. We evaluated the associations of adjuvant chemo with breast-cancer specific survival (BCSS) stratified by tumor size, using multivariate cox models adjusted for tumor size (overall cohort only), age at diagnosis, race, tumor grade, histology, radiation, marital status, income, and rurality. Frequency of chemo use over time was examined by tumor size. Multivariate logistic regression assessed variables associated with chemo administration. Results: In total, we examined data from 8,601 women with stage IA TNBC. Median age at diagnosis was 62 years. Most patients had high-grade tumors (70.1%), ductal histology (92.8%), and received adjuvant chemo (61.6%). Median follow up was 48 months (IQR: 20 – 83). The use of chemo significantly increased during 2010-2019 for both T1b tumors (p for trend = 0.001) and T1c tumors (p for trend < 0.001), with 52.5% and 63% of the patients with T1b and T1c TNBCs receiving chemo in 2010, respectively, compared with 60.5% and 71.1% in 2019. No significant differences were observed in the use of chemo for T1mic (p for trend = 0.567) and T1a (p for trend = 0.637) tumors, with low utilization seen in both cohorts (9.5% overall in T1mic, 22.5% overall in T1a). Variables associated with chemo use included younger age, white race, married status, higher income, more recent diagnosis, higher tumor grade, ductal histology, and larger tumor size (all p < 0.02). Overall, 5-year BCSS was high across T groups ( > 90%; Table). Receiving chemo (vs. none/unknown) was associated with an improved 5-year BCSS (adjusted HR for BCSS = 0.70, p = 0.006). For T1c tumors specifically, adjuvant chemo (vs. none/unknown) was also associated with BCSS (adjusted HR 0.64, p = 0.002). The small number of BCSS events precluded comparisons for other tumor size subgroups. Table shows sample sizes and 5-year BCSS rate by chemo receipt for each T group. Conclusions: In a large population-based cohort of patients with stage IA TNBC, we observed excellent 5-year BCSS outcomes, supporting the good prognosis of this patient population. The use of adjuvant chemo increased over time for T1b and T1c TNBCs, and was associated with improved BCSS for T1c tumors, although evaluation of benefit is limited by the inherent treatment bias in registry data.