Objective To develop a dietary index for Korean adults and assess its association with the risk of metabolic syndrome (MS) in a prospective cohort study. Methods A prospective cohort study included 4,252 male and female Koreans aged 40 to 69 years participating in the Korean Genome Epidemiology Study. All individuals reported having no history of cardiovascular disease, cancer, diabetes mellitus, hypertension, and dyslipidemia and were found not to have 2 or more components of MS, including abdominal obesity, hypertriglyceridemia, hypo-HDL cholesterolemia, and high blood pressure and fasting glucose, at the initial health examination. They completed a food frequency questionnaire administered by trained interviewers. A dietary index (The Korean Diet Index for Adults, KODIA), which reflects a healthy dietary pattern, was constructed on the basis of dietary recommendations and data with regard to the relation to MS risk. Incident MS cases were ascertained through biennial examinations during a follow-up period from April 17, 2003 to March 12, 2009. We used pooled logistic regression models to estimate multivariate relative odds (RO) and 95% confidence interval (CI) for the association between KODIA scores and MS risk. Results The overall mean KODIA score was 48.0±10.0 with a range of 15 to 80 points. After controlling for potential confounding factors, individuals with the highest quintile of KODIA scores ranging 57 to 80 were at a 33% reduced risk of MS (95% CI: 14% to 48%) compared with those with the bottom quintile. In analysis for MS components, such beneficial effects of having high scores of KODIA were found to be driven by inverse associations with all five components. Conclusion The proposed KODIA may be a useful tool for health care professionals to assess dietary status to predict the development of MS and to suggest a healthy eating pattern for Korean adults.