Background: Vitamin D is a key molecular regulator of calcium and phosphate metabolism. Numerous reports have suggested a substantial association between serum vitamin D levels and various chronic diseases. However, few studies have explored the association between diet quality and serum vitamin D levels. We aimed to investigate the association between the Korean Healthy Eating Index (KHEI) score and serum vitamin D levels in Korean adults. Methods: A total of 3,510 adults aged ≥20 years who participated in the 2013–2015 Korea National Health and Nutrition Examination Survey were included in the final analysis. The KHEI scores were categorized into quartiles. One-way analysis of variance was performed to compare serum vitamin D levels according to the KHEI score quartiles. Analysis of covariance was conducted after adjusting for age, sex, body mass index, current smoking, high-risk drinking, physical activity, occupation, and residence. Results: The mean age±standard error of age of the study population was 46.4±0.3 years. The KHEI scores were categorized into quartiles: Q1 (16.0– 55.1), Q2 (55.2–64.5), Q3 (55.2–64.5), and Q4 (64.6–73.1). The multivariable-adjusted mean serum vitamin D level significantly increased as the KHEI score increased (overall P=0.009). Conclusion: The KHEI score is significantly associated with serum vitamin D levels in Korean adults. Considering public health policies, maintaining healthy dietary habits could be crucial for maintaining an adequate serum vitamin D level.