Importance: High blood pressure (BP) during sleep (asleep blood pressure) is associated with an increased risk of cardiovascular disease, but a national prevalence estimate of masked asleep hypertension (high BP while sleeping but without high BP measured in the clinic [clinic BP]) for the United States is lacking. Objectives: To estimate the prevalence of masked asleep hypertension among US adults by using BP thresholds from the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC7) and the 2017 American College of Cardiology-American Heart Association (ACC-AHA) BP guidelines. Design, Setting, and Participants: This cohort analysis pooled data from 3000 participants in 4 US population-based studies that conducted 24-hour ambulatory BP monitoring (ABPM) and 17969 participants in the 2011-2016 National Health and Nutrition Examination Survey (NHANES) without ABPM. Masked asleep hypertension status in NHANES was imputed using a 2-stage multiple imputation process. Data were collected from 2000 to 2016 and analyzed from March 4, 2019, to June 29, 2020. Main Outcomes and Measures: High clinic BP was defined as clinic systolic BP (SBP)/diastolic BP (DBP) of at least 140/90 mm Hg using JNC7 and at least 130/80 mm Hg using 2017 ACC-AHA guidelines. High asleep BP was defined as mean asleep SBP/DBP of at least 120/70 mm Hg for JNC7 and at least 110/65 mm Hg for the 2017 ACC-AHA guidelines. Masked asleep hypertension was defined as high asleep BP without high clinic BP. Results: For the 3000 pooled cohort participants, the mean (SD) age was 52.0 (12.0) years, and 62.6% were women. For the 17 969 NHANES participants, the mean (SD) age was 46.7 (17.5) years, and 51.8% (weighted) were women. The estimated prevalence of masked asleep hypertension among US adults was 18.8% (95% CI, 16.7%-20.8%; 44.4 million US adults) using the JNC7 guideline and 22.7% (95% CI, 20.6%-24.8%; 53.7 million US adults) using the 2017 ACC-AHA guideline criteria. The prevalence of masked asleep hypertension was higher among older adults (aged ≥65 years, 24.4% [95% CI, 20.7%-28.0%]), men (27.0% [95% CI, 24.1%-29.9%]), non-Hispanic Black individuals (28.7% [95% CI, 25.4%-32.0%]), those who were taking antihypertensives (24.4% [95% CI, 21.1%-27.8%]), those who had masked daytime hypertension (44.7% [95% CI, 40.1%-49.3%]), and those with diabetes (27.6% [95% CI, 23.5%-31.8%]), obesity (24.3% [95% CI, 21.8%-26.9%]), or chronic kidney disease (21.5% [95% CI, 17.3%-25.6%]) using the 2017 ACC-AHA guideline. An estimated 11.9% of US adults (28.2 million) had isolated masked asleep hypertension (masked asleep hypertension but without high awake BP) using JNC7 guideline criteria, as did an estimated 13.3% (31.5 million) using 2017 ACC-AHA guideline criteria. Conclusions and Relevance: These findings suggest that the prevalence of masked asleep hypertension is high among US adults. Data are needed on the cardiovascular risk reduction benefits of treating asleep hypertension..