Washtenaw County is a county located in the U.S. state of Michigan. At the 2020 census, the population was 372,258. The county seat and largest city is Ann Arbor. The county was authorized by legislation in 1822 and organized as a county in 1826. Washtenaw County comprises the Ann Arbor Metropolitan Statistical Area. The county is home to the University of Michigan, Eastern Michigan University, Washtenaw Community College, and Concordia University Ann Arbor.

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.