Greencastle is a borough in Franklin County in south-central Pennsylvania, United States. The population was 3,722 at the 2000 census.
Greencastle was founded in 1782 by John Allison. The town was named after a small town in County Donegal, Ireland. It was originally composed of 246 lots. By 1790 there were about sixty 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, Jr.) took his followers to Pennsylvania and formed a Rigdonite Mormon settlement at Greencastle. This settlement had approximately 200 followers. They founded the New Jerusalem settlement between Greencastle and Mercersburg, published the Conochoheague Herald newspaper in Greencastle , and made plans for the construction of a temple. The Rigdonite Mormon settlement at Greencastle only lasted a few years; some former Rigdon followers went to Utah to join Brigham Young, while William Bickerton, who had opposed Rigdon's move to Greencastle, would eventually reorganize the remaining Pennsylvania branch of the Latter Day Saint movement in Pittsburgh as The Church of Jesus Christ (Bickertonite).