To assess the impact of National Cancer Institute socioeconomic status (SES) index on breast cancer-specific survival (BCSS) of nonmetastatic breast cancer patients registered within the Surveillance, Epidemiology and End Results (SEER) census tract-level SES database.The census tract-level SES index is a composite score integrating 7 parameters that assess different dimensions of SES. Women with a nonmetastatic breast cancer diagnosis (stage I-III) diagnosed during 2010-2015 and included in the SEER-SES specialized database were included in the current analysis. Multivariate Cox regression analysis was used to assess the impact of SES index on BCSS.A total of 296,100 women with nonmetastatic breast cancer were included in the current study. The impact of SES index on BCSS was evaluated in the overall cohort of patients through multivariate Cox regression analysis (adjusted for age at diagnosis, race, stage, and breast cancer subtype). Lower SES was associated with worse BCSS (hazard ratio for group 1 [lowest SES group] vs. group 3 [highest SES group]: 1.428; 95% confidence interval, 1.359-1.499; P < .001). Using additional interaction testing within Cox regression models, the impact of SES on BCSS seems to be modified by breast cancer subtype (P for interaction < .001), race (P for interaction = .001), and stage (P for interaction = .015).Lower SES index is associated with worse BCSS. Further efforts need to be directed to improving breast cancer outcomes among women with socioeconomically vulnerable attributes (poverty, lower education, and unemployment).