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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.
Early in the American Civil War, Greencastle and neighboring Franklin County communities raised the 126th Pennsylvania Infantry. In the summer of 1863, the war touched close to home when Confederate General Robert E. Lee and his Army of Northern Virginia invaded lower Pennsylvania in what later became known as the Gettysburg Campaign. From mid-June to early July, those residents of Greencastle that had not fled to safety lived under Confederate rule. On July 2, concurrent with the Battle of Gettysburg in neighboring Adams County, Captain Ulric Dahlgren's Federal cavalry patrol galloped into Greencastle's town square, where they surprised and captured several Confederate cavalrymen carrying vital correspondence from Richmond. After the Battle of Gettysburg, Lee's army began its retreat to Virginia on July 4 and 5. He sent John D. Imboden's cavalry to escort a large wagon train carrying Confederate wounded. The train, nearly 18 miles in length, wound its way through the streets of Greencastle. where few bitter men of the town attacked the wagon train with axes and hatchets. They succeeded in disabling several wagons before Confederate cavalry chased them away.
Following the war, Greencastle grew considerably in the late 19th century during the Industrial Revolution, having several industrial factories built inside the town limits, including the Crowell Manufacturing Company, which constructed farming equipment.
In 1902, Greencastle businessman Philip Baer began a tradition where the town holds a triennial social event known as "Old Home Week." Every three years, Greencastle townspeople and former residents come together in a town-wide reunion to reminisce and fellowship. The most recent Old Home Week Celebration occurred in 2007; the next one is to be held in 2010.