Greencastle is a borough in Franklin County in south-central Pennsylvania, United States. The population was 3,996 at the 2010 census. Greencastle was founded in 1782 by John Allison and also Abigail Siner from the Barkdoll House. The town was named after Greencastle, County Donegal, Ireland. It was originally composed of 246 lots. By 1790 there were about 60 houses in Greencastle, homes to approximately 400 people. The town of Greencastle had grown by the mid-nineteenth century to 1,125 residents.
In 1845, following the succession crisis in the Latter Day Saint movement, Sidney Rigdon (one of the three main contenders along with James Strang and Brigham Young for leadership of the Latter Day Saints following the death of Joseph Smith) took his followers to Pennsylvania and formed a Rigdonite Mormon settlement at Greencastle.