BACKGROUND: Cardiometabolic health has been associated with dementia risk but prospective evidence is limited in China where the burden of cardiometabolic disease and dementia are rising. We investigate the relevance of markers of cardiometabolic health for risk of dementia and brain atrophy. METHODS: China Kadoorie Biobank is a prospective cohort study involving 512,724 adults aged 30-79 years, recruited in 2004-2008 from 10 diverse regions. During ∼12 years' follow-up, 1099 dementia and 1418 brain atrophy cases were recorded through linked death registries and health insurance databases. Cox regression yield adjusted hazard ratios (HRs) for incident dementia and brain atrophy associated with markers of cardiometabolic health. FINDINGS: The incidence rate for dementia and brain atrophy was 19.0 and 25.4 per 100,000 person-years, respectively, higher at older ages, and among males. Prior hypertension (adjusted HR 1.21 [95% CI 1.05-1.39]), diabetes (1.36 [1.13-1.65]) and stroke (2.52 [2.14-2.96]) were associated with higher risks of dementia and brain atrophy (1.30 [1.15-1.47], 1.32 [1.12-1.55], 2.43 [2.12-2.78], respectively). Prior IHD was associated with brain atrophy (1.69 [1.47-1.95]), but not dementia (1.17 [0.97-1.41]). INTERPRETATION: These findings provide evidence of the relevance of markers of cardiometabolic health for dementia among adults in China, highlighting the importance of preventative strategies for cardiometabolic diseases that may lead to benefits for brain health. FUNDING: Wellcome Trust, MRC, BHF, CR-UK, Kadoorie Charitable Foundation, Chinese MoST and NSFC.