Washtenaw County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 344,791. The county seat is Ann Arbor. The county was founded in 1822 and organized as a county in 1826. Washtenaw County comprises the Ann Arbor, MI Metropolitan Statistical Area and is included in the Detroit-Warren-Ann Arbor, MI Combined Statistical Area.