Northumberland is a borough in Northumberland County, Pennsylvania, United States. The population was 3,714 at the 2000 census.
Northumberland was founded in 1772. The land that became Northumberland was purchased from the Iroquois in the first Treaty of Fort Stanwix in 1768, and the village was laid out in 1772. During the American Revolution, Northumberland was evacuated during the Big Runaway in 1778, and only finally resettled in 1784.
Northumberland was the American home of eighteenth-century British theologian, Dissenting clergyman, natural philosopher, educator, and political theorist Joseph Priestley (1733–1804) from 1794 until his death in 1804. The Joseph Priestley House still stands on Priestley Avenue and is a National Historic Landmark on the National Register of Historic Places (NRHP) and a museum administered by the Pennsylvania Historical and Museum Commission. There is one other property in the borough on the NRHP: the Priestley-Forsyth Memorial Library, built by a great grandson of Joseph Priestley. Much of the borough is part of the Northumberland Historic District, which is also on the NRHP.