Plum is a borough in Allegheny County, Pennsylvania, United States. The population was 26,940 at the 2000 census.
Plum is often referred to as Plum Boro or more correctly Plum Borough by locals to distinguish it from its previous status as a township. Plum Borough was founded as Plum Township in 1788 and was reorganized as a borough in 1956.
According to the United States Census Bureau, the borough has a total area of 29.0 square miles (75.1 km²), of which, 28.6 square miles (74.2 km²) of it is land and 0.4 square miles (1.0 km²) of it (1.34%) is water.