Hannastown is an unincorporated community and important historical and archaeological site located in Hempfield Township, Westmoreland County, Pennsylvania, United States. Although the village is not tracked by the Census Bureau, it has been assigned the ZIP code 15635.

Hempfield Township is a township in Westmoreland County, Pennsylvania, United States. The population was 41,585 at the 2020 census, making it the largest suburb in the Pittsburgh Metropolitan Area by population. Its name is derived from Hempfield in Lancaster County, which was formed in 1729 and is named after the hemp fields in the area.