Introduction: Rapid growth of the older adult population requires greater epidemiologic characterization of dementia. We developed national prevalence estimates of diagnosed dementia and subtypes in the highest risk United States (US) population.Methods: We analyzed Centers for Medicare & Medicaid administrative enrollment and claims data for 100% of Medicare fee-for-service beneficiaries enrolled during 2011-2013 and age ≥68 years as of December 31, 2013 (n = 21. 6 million).Results: Over 3.1 million (14.4%) beneficiaries had a claim for a service and/or treatment for any dementia subtype. Dementia not otherwise specified was the most common diagnosis (present in 92.9%). The most common subtype was Alzheimer's (43.5%), followed by vascular (14.5%), Lewy body (5.4%), frontotemporal (1.0%), and alcohol induced (0.7%). The prevalence of other types of diagnosed dementia was 0.2%.Discussion: This study is the first to document concurrent prevalence of primary dementia subtypes among this US population. The findings can assist in prioritizing dementia research, clinical services, and caregiving resources.