Objective: To examine the associations between the allergic triad (asthma, allergic rhinitis, atopic dermatitis) and risk of dementia. Methods: Participants comprised 6,785,948 adults aged ≥40 years who participated in a national health examination in 2009 without any history of dementia before baseline. From 2009 to 2017, we prospectively investigated the associations between physician-diagnosed allergic diseases and risk of incident dementia (all-cause, Alzheimer's disease [AD], vascular dementia [VaD]) ascertained using national health insurance claims data. Results: During 8.1 years of follow-up, 260,705 dementia cases (195,739 AD, 32,789 VaD) were identified. Allergic diseases were positively associated with dementia risk. Compared with individuals without allergic diseases, multivariable hazard ratios (HRs) of all-cause dementia were 1.20 (95% confidence interval [CI] 1.19–1.22) in those with asthma, 1.10 (95% CI 1.09–1.12) with allergic rhinitis, 1.16 (95% CI 1.11–1.21) with atopic dermatitis, and 1.13 (95% CI 1.12–1.14) with any of these allergies. Similarly, individuals with any of the allergic triad had a higher risk of AD (HR 1.16, 95% CI 1.14–1.17) and VaD (HR 1.04; 95% CI 1.01–1.06) than those without any allergic disease. As the number of comorbid allergic diseases increased, the risk of dementia increased linearly (Ptrend ≤ 0.002). Compared with individuals without allergies, those with all three allergic diseases had substantially increased risk of all-cause dementia (HR 1.54, 95% CI 1.35–1.75), AD (HR 1.46; 95% CI 1.25–1.70), and VaD (HR 1.99, 95% CI 1.44–2.75). Interpretation: Asthma, allergic rhinitis, and atopic dermatitis were significantly associated with increased risk of all-cause dementia and subtypes, with dose–effect relationships with the severity of allergic diseases. ANN NEUROL 2022.