Background The impact of the new 2017 American College of Cardiology/American Heart Association (ACC/AHA) hypertension guideline on dementia risk at the population level has not been evaluated. Methods and Results We studied dementia‐free participants in the ARIC (Atherosclerosis Risk in Communities) Study cohort in 1987 to 1989. Hypertension was defined by 2003 Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure and 2017 ACC/AHA guidelines using blood pressure measured at baseline. Dementia was defined using adjudicated consensus diagnoses, informant telephone interviews, and discharge codes from hospitalizations and death certificates. Cox regression estimated hazard ratios (HRs) for dementia and 95% CIs by hypertension categories, adjusting for confounders. Population attributable fraction of dementia was calculated by hypertension categories. Among 13 971 participants followed up for a median of 23 years, 1381 dementia cases were identified. Prevalence of hypertension was 34% and 48%, according to the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure and ACC/AHA criteria, respectively. HRs (95% CIs) compared with normal blood pressure, according to ACC/AHA categories, were 1.35 (1.12–1.61) for elevated blood pressure, 1.28 (1.07–1.52) for hypertension stage 1, and 1.36 (1.18–1.57) for hypertension stage 2. Population attributable fractions (95% CIs) of dementia from blood pressure categories were 3% (1%–5%), 3% (1%–5%), and 9% (4%–14%), respectively. Population attributable fractions (95% CIs) using the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure categories were 6% (3%–9%) for prehypertension, 0% (−2% to 2%) for hypertension stage 1, and 9% (5%–13%) for hypertension stage 2. Conclusions Risk of dementia increased across hypertension categories defined by ACC/AHA guidelines. The population impact on dementia incidence using ACC/AHA categories was similar to the impact of the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure.