Objective: Diabetes is associated with cognitive impairment and greater risk for dementia, but the role of gamma-glutamyltransferase (γ-GT) in dementia has not been elucidated. Methods: We determined incident dementia including Alzheimer’s disease and vascular dementia, analyzing data from participants aged 40 years or older in the National Health Insurance Database, collected by the National Health Insurance Service in Korea, from January 2009 to December 2015. Results: During a median follow-up of 7.6 years, 272,657 participants were diagnosed as having dementia. Higher serum γ-GT was associated with increased risk of dementia (HR = 1.22, 95% CI = 1.20-1.24), and had a strong positive association with early onset dementia (HR = 1.32, 95% CI = 1.24-1.40). An additive impact of higher γ-GT on dementia was observed regardless of glycemic status, and prevalent diabetes with the highest γ-GT quartile had a 1.8-fold increased dementia risk (HR = 1.82, 95% CI = 1.78-1.85). This effect of γ-GT concentration in diabetes was more prominent in individuals with vascular dementia (HR = 1.94, 95% CI = 1.84-2.04). In subgroup analysis, young age, male sex, and relatively healthy subjects with a higher γ-GT quartile had more increased dementia risk. Conclusion: γ-GT concentration as well as glycemic status could be a future risk factor for dementia in the general population.