Purpose: Endometrial cancer is the most commonly diagnosed gynecologic cancer, but no convincing dietary risk factors for this cancer have been identified. Among postmenopausal women, we examined how four key a priori diet quality indices-the Healthy Eating Index-2010, Alternative Healthy Eating Index-2010, alternate Mediterranean Diet, and Dietary Approaches to Stop Hypertension are related to the risk of endometrial cancer in the Women's Health Initiative Clinical Trials and Observational Study. Methods: Our prospective cohort study included 84,415 postmenopausal women with a uterus who completed a food frequency questionnaire at enrollment. Cox proportional hazards models were used to estimate multivariate hazard ratios (HRs) and 95% confidence intervals (CIs) for endometrial cancer associated with increasing quintiles of diet quality index scores. Results: During 13.3years of follow-up, 1392 endometrial cancer cases occurred. After adjustment for known risk factors, having better diet quality (Q5 vs. Q1) was not associated with the risk of endometrial cancer, as evidenced using Healthy Eating Index-2010 (HR: 1.11, 95% CI: 0.93-1.33), Alternative Healthy Eating Index-2010 (HR: 0.98, 95% CI: 0.82-1.17), alternate Mediterranean Diet (HR: 0.98, 95% CI: 0.82-1.17), or Dietary Approaches to Stop Hypertension (HR: 1.00, 95% CI: 0.84-1.19). Conclusions: Diet quality was not associated with endometrial cancer risk in this large cohort of postmenopausal women.