Purpose To estimate the percentage of United States adults who accurately assessed their diet quality (DQ). Design Observational, cross-sectional, nationally representative. Setting National Health and Nutrition Examination Survey, 2015-2018. Sample 9757 (86%) of 11 288 adults aged ≥20 years. Measures Perceived DQ was assessed by asking participants, how healthy is your diet? The five responses included excellent, very good, good, fair, and poor. Measured DQ was assessed using 24-hours dietary recalls scored with 2015 Healthy Eating Index; scores were categorized using a 10-point grading scale. Analysis Matches between perceived and measured DQ that were classified as accurate included: excellent = A, very good = A or B, good = B or C, fair = C or D, and poor = D or F. All others were classified as inaccurate. Analyses included descriptive statistics and multivariable logistic regression for complex survey designs. Results 63% of adults perceived their DQ as very good or good while 70% scored DQ grades of F. Overall, 15% of adults accurately assessed their DQ with 96% accuracy in the poor perception group and <23% in the other 4 groups. Overall, 75% of adults overrated their DQ. Females, adults with lower educational attainment, and those with low food security were more likely to accurately assess their DQ. Conclusion Adults cannot accurately assess their DQ except for those perceiving their DQ as poor, and the majority overrate their DQ.