Lifestyle factors are important for cancer survival. However, empirical evidence regarding the effects of dietary changes on mortality in breast cancer survivors is sparse. The objective was to examine the associations of changes in overall diet quality, indicated by the Healthy Eating Index (HEI)-2010 score, with mortality in breast cancer survivors. This was a prospective cohort study from September 1993 through September 30, 2015. This study included 2,295 postmenopausal women who were diagnosed with invasive breast cancer and completed a food frequency questionnaire both before and after the diagnosis of breast cancer in the Women’s Health Initiative. The HEI-2010 score (maximum score of 100) was calculated based on consumption of 12 dietary components. The outcomes were mortality from all causes, breast cancer, and causes other than breast cancer. Multivariable Cox proportional hazards models were used to estimate adjusted hazard ratios of mortality from all causes, breast cancer, and other causes. Over 12 years of follow-up, 763 deaths occurred. Compared with women with relatively stable diet quality (±14.9% change in HEI-2010 score), women who decreased diet quality (≥15% decrease in HEI-2010 score) had a higher risk of death from breast cancer (adjusted hazard ratio 1.66, 95% CI 1.09 to 2.52). Increased diet quality (≥15% increase in HEI-2010 score) was not significantly associated with lower risk of death. These associations persisted after additional adjustment for change in body mass index. Among women with breast cancer, decreased diet quality after breast cancer diagnosis was associated with higher risk of death from breast cancer.