Objective Gestational diabetes mellitus (GDM) is a strong predictor of postpartum prediabetes and the transition to overt type 2 diabetes (T2DM). Decreased blood and tissue levels of magnesium are related to high blood pressure, systemic inflammation, insulin resistance, and the incidence of T2DM. Although many reports indicate that low magnesium status is correlated with deteriorated glucose tolerance, serum magnesium level and the risk for T2DM in women with a GDM history has not been evaluated. Methods We analyzed postpartum serum magnesium levels and the development of prediabetes and T2DM in women with prior GDM according to the ADA criteria using the Korean National Diabetes Program (KNDP) GDM cohort. Results During the mean follow-up of 15.6±2.0 months after screening, 116 women were divided into three groups according to glucose tolerance status. Finally, eight patients (6.8%) were diagnosed with T2DM, 59 patients (50.9%) with prediabetes, and 49 patients (42.2%) with normal glucose tolerance (NGT) after follow-up. The T2DM group had the lowest serum magnesium level (1.30 [1.25-1.35] mEq/L) at postpartum, but the prediabetes group (1.4 [1.30-1.40] mEq/L) and the NGT group (1.4 [1.30-1.40] mEq/L) showed no significant differences (P=0.519). A multiple regression analysis showed that postpartum serum magnesium was a significant predictor for T2DM (odds ratio, 0.273×10-5, 95% confidence interval, 0.00-0.609). Conclusion Postpartum low serum magnesium status is a potent risk factor for the development of T2DM in women with a history of GDM.