Background <p>Evidence on the temporal dynamics of dementia risk before incident diabetes remains limited. We aimed to investigate the temporal pattern of dementia risk before and after incident diabetes, compared with matched controls.</p> Methods <p>Case-control and cohort analyses were conducted by using data from the UK Biobank. Propensity score matching was performed to match participants with and without diabetes. Conditional logistic regression was adopted in case-control analysis to estimate dementia risk before incident diabetes. Cox proportional-hazard regression models were adopted in cohort analysis to estimate dementia risk following a diabetes diagnosis.</p> Results <p>A total of 44,166 participants with diabetes and 132,498 matched diabetes-free participants were included. During the 10-year period before diabetes, dementia occurred in 344 (0.78%) participants who later had diabetes and 530 (0.40%) diabetes-free participants. Compared to participants without diabetes, the dementia risk was significantly higher in participants with diabetes (Odds Ratio [OR]: 2.14, 95% Confidence Interval [CI]: 1.83–2.50), with the ORs gradually increasing as the time approached the diagnosis date. The adjusted OR was 3.86 (95% CI: 3.11–4.80) for dementia in the 2 years before diabetes. After a diabetes diagnosis, a higher risk for dementia remained in participants with incident diabetes (Hazard Ratio [HR]: 1.82, 95%CI: 1.68–1.97). Similar results were found for Alzheimer’s disease and vascular dementia (VD), with VD showing a risk of over 6-fold.</p> Conclusions <p>Dementia risk starts to increase 10 years before diabetes, and the 2-year period preceding diagnosis might represent a high-risk period that calls for closer cognitive monitoring for vulnerable individuals.</p>