As population aging accelerates, healthcare expenditures are steadily increasing. In Korea, poor diet quality has been linked to various chronic diseases, but evidence on its association with actual medical costs remains limited.This cross-sectional study examined the association between diet quality, measured by the Korean Healthy Eating Index (KHEI), and med-ical expenditures among Korean adults.We used data from the Korea Na-tional Health and Nutrition Examination Survey (2016–2021). Adults aged ≥20 years with complete data on diet, sociodemographics, and healthcare use were included. Medical costs were estimated from self-reported service use and converted to USD. KHEI scores were categorized into quartiles. Multivariable linear regression was used to assess the association between KHEI quartiles and log-transformed costs. Subgroup analyses were conducted by age (<57 vs. ≥57 years), and sensitivity analyses treated KHEI as a contin-uous variable. A two-part model addressed skewed, zero-inflated cost data.Compared to Q1, participants in Q4 had significantly lower inpatient (β = -0.080; 95% CI: -0.139 to -0.020) and total costs (β = -0.086; 95% CI: -0.144 to -0.027). In the younger group, Q4 was associated with lower total costs (β = -0.115; 95% CI: -0.198 to -0.031). Higher continuous KHEI scores were also linked to lower costs. In the two-part model, Q4 par-ticipants had a higher probability of incurring any cost but lower conditional costs (Q3 vs. Q1: β = -0.173; Q4 vs. Q1: β = -0.160; both p < 0.05).Higher adherence to a healthy dietary pattern, measured by the Korean Healthy Eating Index (KHEI), was associated with lower inpatient and total healthcare expenditures after adjusting for sociodemographic and health factors. Although participants with higher KHEI scores were older and had more chronic conditions, they incurred lower medical costs, possibly due to better disease management. The inverse association was significant mainly among younger adults, suggesting age-specific differences in how diet affects healthcare use.Higher diet quality was associated with reduced healthcare costs in Korean adults, especially among younger individuals. Promoting healthy eating may help lower economic burdens in aging soci-eties.