Plainfield is a town in Guilford, Liberty, and Washington townships, Hendricks County, Indiana, United States. The population was 27,631 at the 2010 census.
In 1822 a tract of land which included the area now known as Plainfield was obtained by Jeremiah Hadley of Preble County, Ohio. Ten years later he sold it to his son, Elias Hadley. Levi Jessup and Elias Hadley laid out the town in 1839. Plainfield was incorporated as a town in 1839. The town got its name from the early Friends (Quakers) who settled around the area and established several meetinghouses throughout the county, including the important Western Yearly Meeting of Friends in Plainfield. The Friends were "plain" people, and thus the name Plainfield. The high school continues to honor the Quakers, using the name for the school's mascot.
Plainfield has long been associated with the national road, U.S. Route 40, which goes through town as "Main Street." One incident which brought Plainfield national attention occurred in 1842 when former President Martin Van Buren was spilled deliberately from his stage coach into the thick mud of the highway. The practical joke came as a result of Van Buren's vetoing a bill from Congress to improve the highway, a move which angered Western settlers. When Van Buren came through Plainfield on a swing to shore up his popularity for the 1844 election, a group of perpetrators set up the incident. The elm tree whose roots caused the president's carriage to topple became known as the Van Buren Elm. An elementary school near this site is named Van Buren Elementary School. In the 1980s Plainfield became the headquarters of the Islamic Society of North America.
The Hendricks County Bridge Number 316, Plainfield Historic District, and THI and E Interurban Depot-Substation are listed on the National Register of Historic Places.