Adair is an unincorporated community and census-designated place in McDonough County, Illinois, United States. It is part of New Salem Township. As of the 2020 census, Adair had a population of 212.Adair was laid out in August 1870 under the name of "Reedyville"; though the name has never been officially changed, the location is known as Adair because that name was given to the post office. Adair is in eastern McDonough County, along U.S. Route 136.