Plainfield is a town in Sullivan County, New Hampshire, United States. At the 2010 census, the town had a total population of 2,364. The town is home to the Helen Woodruff Smith Bird Sanctuary and Annie Duncan State Forest.

The central settlement in town, where 205 people resided at the 2010 census, is defined as the Plainfield census-designated place (CDP) and is located along New Hampshire Route 12A. Plainfield includes the village of Meriden, home to Kimball Union Academy, a private preparatory school.

The town was first settled by a group from Plainfield, Connecticut. It was one of the towns incorporated by Colonial Governor Benning Wentworth in 1761, at the beginning of the reign of King George III. A part of Plainfield known as Meriden Parish, named for the farm of Massachusetts Governor Jonathan Belcher, became the site of Kimball Union Academy, built in 1813.

Plainfield lies on the northern edge of the enclave known as the Cornish Art Colony, which existed between 1885 and 1930. Maxfield Parrish painted the stage backdrop in the Plainfield Town Hall. Parrish purchased land close to his parents' estate in 1898, at the age of 28, and relocated after leaving Philadelphia, Pennsylvania. He designed and built his home, The Oaks, over several years, eventually also designing and building an elaborate studio some distance behind the house. Parrish lived in Plainfield for 67 years, with the exception of a several-month period in Arizona, and a nine-month period when he resided and worked in New York City. In 1910, Ernest Harold Baynes founded the Meriden Bird Club, the first institution of its type in the nation.

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