Ann Arbor is a city in and the seat of government of Washtenaw County, Michigan, United States. The 2020 census recorded its population to be 123,851, making it the fifth-most populous city in Michigan. Located on the Huron River, Ann Arbor is the principal city of its metropolitan area, which encompasses all of Washtenaw County and had 372,258 residents in 2020. Ann Arbor was founded in 1824 by John Allen and Elisha Rumsey.