Plainfield is a town in Windham County, Connecticut, United States. The population was 15,405 at the 2010 census. The town comprises four villages: Plainfield (south, ZIP code 06374), Moosup (northeast, 06354), Wauregan (northwest, 06387), and Central Village (west, 06332). Each village has their own respective United States Post Office and fire department. The entire town is serviced by the 860 area code.

Plainfield was incorporated in 1699 as the town of Quinebaug and renamed the following year to its current name. Plainfield proved to be an industrial heavyweight in the 19th and early 20th centuries due largely to the ability of the Moosup and Quinebaug Rivers to provide power to the different mills throughout town. Combined with the mills, Plainfield had incredible commerce for a small town because four railways from Hartford, New York, Boston, and Providence intersected within the town. There was also a highly acclaimed school on Academy Hill, which is how the present Academy Hill Road got its name.

Sites in Plainfield that are listed on the National Register of Historic Places are:

Plainfield is located at: 41.6775 N, 71.9220 W.

According to the United States Census Bureau, the town has a total area of 43.0 square miles (111.5 km²), of which, 42.3 square miles (109.5 km²) of it is land and 0.8 square miles (2.0 km²) of it (1.81%) is water. From the southernmost point, Plainfield is 20 miles inland of the Long Island Sound and Block Island Sound (Atlantic Ocean). Plainfield is the southernmost town of Windham County. The Quinebaug River generally forms the western border of the town. The Moosup River feeds the Quinebaug and runs across the northern portion of town.

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