Greencastle is a city in Greencastle Township, Putnam County, Indiana, United States, and the county seat of Putnam County. It was founded in 1821 by English American Ephraim Dukes on a land grant. He named the settlement for his hometown of Greencastle, Pennsylvania. 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.