Several studies have observed an inverse association between serum magnesium and fasting glucose levels. However, there have been sparse data on Koreans. The present study was designed to evaluate the association between serum magnesium and fasting serum glucose levels in Korean adults. A total of 949 Korean adults who visited an outpatient at a university hospital were included in this study. We compared mean values of clinical data according to the tertile of serum magnesium level using analysis of covariance. Association between serum magnesium and fasting glucose levels was evaluated using multiple regression analysis. Odds ratio (OR) was calculated to determine the association between hypomagnesemia and abnormal glucose metabolism. Fasting serum glucose levels decreased significantly from the lowest to the highest tertile of serum magnesium level (108.0±1.4, 106.5±2.1, 102.5±1.7 mg/dl, respectively, p=0.015), whereas blood pressure and lipid profile showed no significant associations. Serum magnesium level was negatively associated with fasting serum glucose level (β=-0.114, p=0.001). Participants with hypomagnesemia had significantly higher OR for abnormal glucose metabolism compared to those with normomagnesaemia (OR=2.28, 95%CI 1.29-4.02). There was a negative association between serum magnesium and fasting glucose levels in Korean adults. In addition, hypomagnesemia was associated with abnormal glucose metabolism.