Background: Black women have worse breast cancer outcomes compared to their White counterparts, even for hormone receptor positive (HR+) HER2 negative (HER2-) breast cancer (BC). This tumor type is usually treated with 5-10 years of adjuvant endocrine therapy (ET) to reduce risk of BC recurrence and improve survival. We investigated the impact of adherence to ET on racial differences in survival for HR+ HER2- BC survivors. Methods: We conducted a population-based, retrospective cohort study using the Surveillance, Epidemiology, and End Results (SEER)-Medicare database, which links demographic and clinical data with cause of death for persons with cancer and Medicare claims for covered health services. We examined 9,039 women with HR+ HER2- stage I-III BC diagnosed between 2007 and 2016, who received primary breast surgery and initiated ET. Adherence to ET was measured using the medication possession ratio (MPR), which assesses the proportion of time a patient has medication available via filled prescriptions. As in established literature, we defined ET adherence as MPR > = 80%. Causal mediation analysis was performed to measure to impact of adherence on survival disparities between Black and White BC survivors. Results: Mean age at diagnosis was 72 years, 6.5% were Black, and 20.9% were dual eligible for Medicaid (a surrogate for socioeconomic hardship). Most had stage I or II breast cancer (69.3%) and mastectomy as primary surgery (69.7%). Nineteen percent received chemotherapy and 62.5% received radiation. In univariate analyses, factors associated with ET adherence were Black race (OR 0.62, 95% CI 0.54-0.75), insured by Medicaid (OR 0.72, 95% CI 0.65-0.81), being married (OR 1.14, 95% CI 1.03-1.25), and increased comorbidities (Charlson score > = 2) (OR 0.88, 95% CI 0.78-0.99). Age, BC stage, type of surgery, and receipt of chemotherapy or radiation were not associated with ET adherence. In multivariable models adjusting for age, race, marital status, Medicaid eligibility, and Charlson score, Black race and Medicaid eligibility remained inversely associated with ET adherence (OR 0.70, 95% CI 0.59-0.83 and OR 0.77, 95% CI 0.69-0.87 respectively). Among those who were adherent to ET, Black women had 43% higher risk of mortality compared to White women (HR 1.43, 95% CI 1.17-1.76). In mediation analysis, 4.25% of the observed survival disparity between Black and White women can be explained by differences in ET adherence. Conclusions: We identified racial disparities in ET adherence and overall survival among women with BC. Adherence to ET accounted for a small percentage of racial disparities in survival. Further research is needed to delineate other factors contributing to ET non-adherence among Black women and to investigate additional factors contributing to known disparities in BC survival.