Few studies have evaluated whether adherence to dietary recommendations is associated with mortality among cancer survivors. In breast cancer survivors, we examined how postdiagnosis Healthy Eating Index (HEI)-2005 scores were associated with all-cause and cause-specific mortality. Our prospective cohort study included 2,317 postmenopausal women, ages 50 to 79 years, in the Women’s Health Initiative’s Dietary Modification Trial (n = 1,205) and Observational Study (n = 1,112), who were diagnosed with invasive breast cancer and completed a food frequency questionnaire after being diagnosed. We followed women from this assessment forward. We used Cox proportional hazards models to estimate multivariate-adjusted HRs and 95% confidence intervals (CI) for death from any cause, breast cancer, and causes other than breast cancer, according to HEI-2005 quintiles. Over 9.6 years, 415 deaths occurred. After adjustment for key covariates, women consuming better quality diets had a 26% lower risk of death from any cause (HRQ4:Q1, 0.74; 95% CI, 0.55-0.99; Ptrend = 0.043) and a 42% lower risk of death from non-breast cancer causes (HRQ4:Q1, 0.58; 95% CI, 0.38-0.87; Ptrend = 0.011). HEI-2005 score was not associated with breast cancer death (HRQ4:Q1, 0.91; 95% CI, 0.60-1.40; Ptrend = 0.627). In analyses stratified by tumor estrogen receptor (ER) status, better diet quality was associated with a reduced risk of all-cause mortality among women with ER(+) tumors (n = 1,758; HRQ4:Q1, 0.55; 95% CI, 0.38-0.79; Ptrend = 0.0009). Better postdiagnosis diet quality was associated with reduced risk of death, particularly from non-breast cancer causes. Breast cancer survivors may experience improved survival by adhering to U.S. dietary guidelines.