Plainfield is a town on the northwestern edge of Hampshire County, Massachusetts, United States, about 25 miles east of Pittsfield and 30 miles northwest of Northampton. The population was 633 at the 2020 census. It is part of the Springfield, Massachusetts Metropolitan Statistical Area.

Hampshire County is a historical and judicial county located in the U.S. state of Massachusetts. As of the 2020 census, the population was 162,308. Its most populous municipality is Amherst, its largest town in terms of landmass is Belchertown, and its traditional county seat is Northampton. The county is named after the county Hampshire, in England. Hampshire County is part of the Springfield, MA Metropolitan Statistical Area. Together with Hampden County, Hampshire County municipalities belong to the Pioneer Valley Planning Commission.