Background: Breast cancer patients of low socioeconomic status (SES) have worse survival than more affluent women and are also more likely to undergo surgery in low-volume facilities. Since breast cancer patients treated in high-volume facilities have better survival, regionalizing the care of low SES patients toward high-volume facilities might reduce SES disparities in survival. Objective: We leverage a natural experiment in New York state to examine whether a policy precluding payment for breast cancer surgery for New York Medicaid beneficiaries undergoing surgery in low-volume facilities led to reduced SES disparities in mortality. Research Design: A multivariable difference-in-differences regression analysis compared mortality of low SES (dual enrollees, Medicare-Medicaid) breast cancer patients to that of wealthier patients exempt from the policy (Medicare only) for time periods before and after the policy implementation. Subjects: A total of 14,183 Medicare beneficiaries with breast cancer in 2006–2008 or 2014–2015. Measures: All-cause mortality at 3 years after diagnosis and Medicaid status, determined by Medicare administrative data. Results: Both low SES and Medicare-only patients had better 3-year survival after the policy implementation. However, the decline in mortality was larger in magnitude among the low SES women than others, resulting in a 53% smaller SES survival disparity after the policy after adjustment for age, race, and comorbid illness. Conclusion: Regionalization of early breast cancer care away from low-volume centers may improve outcomes and reduce SES disparities in survival.