Plainfield High School is a comprehensive community four-year public high school that serves students in ninth through twelfth grades from Plainfield, in Union County, New Jersey, United States. The school is part of the Plainfield Public School District, one of New Jersey's 31 former Abbott districts. Plainfield High School was established in 1857, making it the second-oldest high school in New Jersey. The school has been accredited by the Middle States Association of Colleges and Schools Commission on Elementary and Secondary Schools since 1928.