BACKGROUND: Flooding has emerged as the most prevalent natural disaster, impacting billions of individuals worldwide. However, the long-term effects of flooding exposure on dementia remain unclear. METHODS: With a nested case-control design, a risk-set sampling method was used to match cases and controls. Annual cumulative flooding exposure was calculated for each participant. The associations between flooding exposure and incident dementia were assessed using conditional logistic regression models. RESULTS: Here we show that the risk of flood-related incident dementia is the strongest in the current year and diminished over a span of 6 years. In the fully adjusted model, the cumulative odds ratios (OR) are 1.28 (95% confidence interval [CI]: 1.25-1.31) for any dementia, 1.44 (95% CI: 1.36-1.53) for Alzheimer's disease and 1.65 (95% CI: 1.48-1.83) for vascular dementia, associated with per unit increase in annual cumulative flooding exposure over lag 0-6 years. Participants under the age of 65 years (OR: 1.39, 95% CI: 1.33-1.46) and female participants (OR: 1.41, 95% CI: 1.33-1.49) exhibit a higher risk of incident dementia compared to those aged 65 years and older (OR: 1.25, 95% CI: 1.21-1.28) and male participants (OR: 1.26, 95% CI: 1.23-1.30), respectively. Similar effect estimates are observed in the stratified analyses of Alzheimer's disease according to genetic factors. CONCLUSIONS: This study provides robust epidemiological evidence supporting the link between floods and an increased risk of dementia. These findings enhance the understanding of the long-term consequences of flood exposure.