Greencastle is a city in Greencastle Township, Putnam County, Indiana, United States, and the county seat of Putnam County. Founded in 1821 by Ephraim Dukes on a land grant, Greencastle was a village or town operating under authority of the Putnam County commissioners until March 9, 1849, when it became a town by special act of the local legislature. The city became the county seat of Putnam County. Located near Interstate 70, Greencastle is also well known as being the location of DePauw University.