Aims/hypothesis: Mendelian randomisation studies have not shown a clear causal effect of high plasma glucose on the risk of Alzheimer’s disease. We tested the hypothesis that high plasma glucose caused by genetic variation has a causal effect on the risk of unspecified dementia, Alzheimer’s disease and vascular dementia in the general population. Methods: A Mendelian randomisation design was used with data from 115,875 individuals from the Copenhagen General Population Study and the Copenhagen City Heart Study. Findings for Alzheimer’s disease were validated in a two-sample Mendelian design with 455,258 individuals, including 71,880 individuals with Alzheimer’s disease or a parent with Alzheimer’s disease. Results: In observational multifactorial-adjusted analyses, HRs were 1.15 (95% CI 1.01, 1.32; p = 0.039) for unspecified dementia, 0.91 (95% CI 0.79, 1.06; p = 0.22) for Alzheimer’s disease and 1.16 (95% CI 0.86, 1.55; p = 0.34) for vascular dementia in individuals with a glucose level higher than 7 vs 5–6 mmol/l. Corresponding HRs in individuals with vs without type 2 diabetes were 1.42 (95% CI 1.24, 1.63; p < 0.001), 1.11 (95% CI 0.95, 1.29; p = 0.18) and 1.73 (95% CI 1.31, 2.27; p < 0.001). In genetic causal analyses, a 1 mmol/l higher plasma glucose level had RRs of 2.40 (95% CI 1.18, 4.89; p = 0.016) for unspecified dementia, 1.41 (95% CI 0.82, 2.43; p = 0.22) for Alzheimer’s disease and 1.20 (95% CI 0.82, 1.75; p = 0.36) for vascular dementia. Summary-level data from the Meta-Analyses of Glucose and Insulin-related Traits Consortium (MAGIC) combined with a consortium of the Alzheimer’s Disease Sequencing Project (ADSP), the International Genomics of Alzheimer’s Project (IGAP), the Alzheimer’s Disease Working Group of the Psychiatric Genomics Consortium (PGC-ALZ) and the UK Biobank (UKB) gave an RR for Alzheimer’s disease of 1.02 (95% CI 0.92, 1.13; p = 0.42), and this consortium combined with Copenhagen studies gave an RR for Alzheimer’s disease of 1.03 (95% CI 0.93, 1.13; p = 0.36). Conclusions/interpretation: Observational and genetically high plasma glucose are causally related to the risk of unspecified dementia, but not to Alzheimer’s disease or vascular dementia.