Hannastown is an unincorporated community and important historical and archaeological site located in Hempfield Township, Westmoreland County, Pennsylvania. Although the village is not tracked by the Census Bureau, it has been assigned the ZIP code 15635.
The village was founded in 1773 as the seat of the newly created Westmoreland County, and was known at the time as "Hanna's Town". It was located along Forbes Road, the main route into the Ohio Country from eastern Pennsylvania, and named for Robert Hanna, an early settler whose tavern also served as Westmoreland County's first courthouse. Hannastown was settled primarily by Irish and Scotch-Irish, though the surrounding area was mostly Pennsylvania Dutch.
On July 13, 1782, in one of the final actions of the American Revolutionary War, the settlement was destroyed by a force of The King's 8th Regiment out of Fort Niagara "The King's, or 8th Regiment - Detroit Garrison" and British-allied American Indians led by Guyasuta. The county government was moved to Newtown, which later became known as Greensburg. The village was rebuilt, but after Forbes Road was rerouted through Greensburg, the settlement grew little, and eventually most of it became farmland.