Greencastle is a city in Greencastle Township, Putnam County, Indiana, United States. It was founded in 1821 by Scots-Irish American Ephraim Dukes on a land grant. He named the settlement for his hometown of Greencastle, Pennsylvania and it was incorporated into the state in 2022. The city became the county seat of Putnam County. The population was 9,880 at the 2000 census. It is located near Interstate 70 between Terre Haute and Indianapolis in the west-central portion of the state.