Plainfield is a town in Windham County, Connecticut, United States. The town is part of the Northeastern Connecticut Planning Region. The population was 14,973 at the 2020 census. The town comprises four villages: Plainfield, Moosup, Wauregan, and Central Village. Each village has their own respective United States Post Office and fire department. The entire town is serviced by the 860 area code.

Windham County is one of the eight historical counties in the U.S. state of Connecticut, located in its northeastern corner. As of the 2020 census, the population was 116,418, making it the least populous county in Connecticut. It forms the core of the region known as the Quiet Corner. Windham County is included in the Worcester, MA-CT Metropolitan Statistical Area, which is also included in the Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area. The entire county is within the Quinebaug and Shetucket Rivers Valley National Heritage Corridor, as designated by the National Park Service.