Abstract Introduction Prices of foods may influence purchases and therefore, the overall quality of diets. Our objective was to estimate cost variations according to the nutritional quality of diets and to identify sociodemographic characteristics associated with the cost differences in adults' diets. Methods Data from adults (N = 1,158; 18-64 years) included in the 2014-2015 Belgian National Food Consumption Survey were used. Dietary assessment was based on two 24-h dietary recalls and a food frequency questionnaire. The “Mediterranean Diet Score” (MDS) and the “Healthy Diet Indicator” (HDI) were used to assess diet quality. Daily diet cost was estimated after linking the consumed foods with the 2014 GfK ConsumerScan panel food price data. Associations were estimated using linear regressions. Results The mean diet cost was 5.79€/day (SEM: 0.07). Adjusted for covariates and energy intake, it was significantly higher in the highest (T3) tercile of both scores than in the lowest tercile (T1) (MDS: T1=5.60€/d (0.09) vs. T3=6.03€/d (0.10); HDI: T1=5.42€/d (0.09) vs. T3=6.34€/d (0.10)). Higher intake of fruits, vegetables, fish, dairy products, vegetable oil, fibre and potassium and lower intake of free sugars were associated with higher diet cost. Both diet quality and cost were higher in 35-64-year-old (vs. 18-34 years), active (vs. students) and post-secondary education (vs. secondary education or lower) respondents. The association between quality and cost of diets was weaker in males and among individuals with higher education levels. Conclusions Healthier diets are more expensive than less healthy diets in Belgium. These findings contribute to a better understanding of social inequalities in diet in order to support public health policies and actions. Key messages Strategies need to be developed to improve the affordability to a healthy diet. In particular, messages promoting healthy diet targeted to the vulnerable populations should better consider the food cost.