An integrative polygenic risk score (iPRS) capturing the neurodegenerative and vascular contribution to dementia could identify high-risk individuals and improve risk prediction. We developed an iPRS for dementia (iPRS-DEM) in Europeans (aged 65+), comprising genetic risk for Alzheimer's disease (AD) and 23 vascular or neurodegenerative traits (excluding apolipoprotein E [APOE]). iPRS-DEM was evaluated across cohorts comprising older community-dwelling people (N = 3702), a multi-ancestry biobank (N = 130,797 Europeans; 105,404 non-Europeans), and dementia-free memory clinic participants (N = 2032). iPRS-DEM was associated with dementia risk independently of APOE in the elderly (subdistribution hazard ratio [sHR]_per1SD= 1.15, 95% confidence interval [CI]: 1.03 to 1.28), which generalized to Europeans (EUR-sHR_per1SD= 1.28, 95% CI: 1.09 to 1.51]), East-Asians (EAS-sHR_per1SD= 5.29, 95% CI: 1.43 to 34.36), and memory-clinic participants (sHR_per1SD= 1.25, 95% CI: 1.11 to 1.42). Prediction was comparable to clinical risk factors in older community-dwelling people, with improved performance among memory-clinic patients. Risk stratification was enhanced by defining four genetic risk groups with iPRS-DEM and APOE ε4, reaching five-fold increased risk in APOE ε4+/iPRS-DEM+ memory-clinic participants. Alongside APOE ε4, iPRS-DEM may refine risk stratification for the enrichment of dementia clinical trials and prevention programs. iPRS-DEM reflects neurodegenerative and vascular contribution to dementia. We show iPRS-DEM captures additional dementia genetic risk beyond APOE and AD-PRS. iPRS-DEM, in combination with APOE ε4, shows promise for dementia risk stratification. Our results generalize across both population-based and memory-clinic settings. We show transportability of iPRS-DEM to East Asian ancestry.