Objective: To understand the role of racial residential segregation on Black-White disparities in breast cancer presentation, treatment, and outcomes. Summary of Background Data: Racial disparities in breast cancer treatment and outcomes are well documented. Black individuals present at advanced stage, are less likely to receive appropriate surgical and adjuvant treatment, and have lower overall and stage-specific survival relative to White individuals. Methods: Using data from the Surveillance, Epidemiology, and End Results program, we performed a retrospective cohort study of Black and White patients diagnosed with invasive breast cancer from 2005 to 2015 within the 100 most populous participating counties. The racial index of dissimilarity was used as a validated measure of residential segregation. Multivariable regression was performed, predicting advanced stage at diagnosis (stage III/ IV), surgery for localized disease (stage I/II), and overall stage-specific survival. Results: After adjusting for age at diagnosis, estrogen/progesterone receptor status, and region, Black patients have a 49% greater risk (relative risk [RR] 1.49 95% confidence interval [CI] 1.27, 1.74) of presenting at advanced stage with increasing segregation, while there was no observed difference in Whites (RR 1.04, 95% CI 0.93, 1.16). Black patients were 3% less likely to undergo surgical resection for localized disease (RR 0.97, 95% CI 0.95, 0.99) with increasing segregation, while Whites saw no significant difference. Black patients had a 29% increased hazard of death (RR 1.29, 95% CI 1.04, 1.60) with increasing segregation; there was no significant difference among White patients. Conclusions: Our data suggest that residential racial segregation has a significant association with Black-White racial disparities in breast cancer. These findings illustrate the importance of addressing structural racism and residential segregation in efforts to reduce Black-White breast cancer disparities.