Ann Arbor is a city in and the county seat 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. It is the principal city of the Ann Arbor metropolitan area, which encompasses all of Washtenaw County and had 372,258 residents in 2020. Ann Arbor is also included in the Metro Detroit combined statistical area and the Great Lakes megalopolis.

The Great Lakes megalopolis consists of a bi-national group of metropolitan areas in North America largely in the Great Lakes region. It extends from the Midwestern United States in the south and west to western Pennsylvania and Western New York in the east and northward through Southern Ontario into southwestern Quebec in Canada. It is the most populated and largest megalopolis in North America.