Objective: Gamma-glutamyl transferase (GGT) has been reported to be associated with oxidative stress and inflammatory reaction. Recently, a number of studies have focused on the association between variability in biologic parameters and various diseases. So we investigated the association between GGT variability and dementia risk for the first time in diabetes (DM) population. Methods: The Korean National Health Insurance Service datasets of Claims and Health Check-ups from 2004 to 2016 were used for this retrospective longitudinal study. The risk of incident dementia (all-cause dementia, Alzheimer disease, and vascular dementia) was analyzed according to the quartiles of GGT variability in ≥ 40-year-old-DM individuals without baseline dementia. Results: The fully-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause dementia increased in the higher quartiles of GGT variability [HR (95% CI): Q2, 1.04 (1.01-1.07); Q3, 1.09 (1.06-1.13); Q4, 1.23 (1.19-1.26)]. The association between increased GGT variability and dementia risk was more prominent in men and individuals aged < 60 years in the subgroup analysis (P for interaction < 0.001). Conclusion: It demonstrated that higher GGT variability was significant predictive factors for the development of dementia, independent of other risk factors in DM patients.