Derry is a borough in Westmoreland County in Pennsylvania, 45 miles (72 km) east of Pittsburgh. The Borough of Derry, consisting of the town area, should not be confused with Derry Township, which is a separate municipality surrounding the borough. The population was 2,637 at the 2020 census.

Westmoreland County is a county in the Commonwealth of Pennsylvania, United States, located in the Pittsburgh Metropolitan Statistical Area. As of the 2020 census, the population was 354,663. The county seat is Greensburg and the most populous community is Hempfield Township. It is named after Westmorland, a historic county of England. The county is part of the Southwest Pennsylvania region of the state.