Plainsboro Township is a township in Middlesex County, New Jersey, United States. As of the United States 2000 Census, the township population was 20,215. Plainsboro was incorporated as a township on May 6, 1919. Prior to that date, all lands north of Plainsboro Road and Dey Road were part of South Brunswick Township and all lands south of Plainsboro Road and Dey Road were part of Cranbury Township. Inadequate school facilities were the catalyst for creating the township. Plainsboro Township Go… (More on Plainsboro)