Plainfield is a town in Sullivan County, New Hampshire, United States. As of 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.