is a borough in Allegheny County, Pennsylvania, United States. The population was 26,940 at the 2000 census.
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.