Plainfield is a town in Sullivan County, New Hampshire, United States. At the time of the 2020 census, the town had a population of 2,459. The town is home to the Helen Woodruff Smith Bird Sanctuary and Annie Duncan State Forest. The village of Plainfield, where 178 people resided at the 2020 census, is defined as the Plainfield census-designated place and is located in the western part of the town along New Hampshire Route 12A.