Arnold is a city in Westmoreland County, Pennsylvania, United States. It is part of the Greater Pittsburgh metropolitan area. The population was 5,157 at the 2010 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.