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-largest city in Michigan. It is the principal city of the Ann Arbor Metropolitan Statistical Area, which encompasses all of Washtenaw County. Ann Arbor is also included in the Greater Detroit Combined Statistical Area and the Great Lakes megalopolis, the most populated and largest megalopolis in North America.