Derry is a borough in Westmoreland County in the Commonwealth of 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. In 1900, the population numbered 2,347, and in 1910, 2,954. The population was 2,991 at the 2000 census.