Northumberland County is a county located in the U.S. state of Pennsylvania. It was formed in 1772 from parts of Lancaster, Berks, Bedford, Cumberland, and Northampton Counties and named for the county of Northumberland in northern England. As of 2010, the population was 94,528. Its county seat is Sunbury. Northumberland County is a sixth class county according to the Pennsylvania's County Code.
Among its famous residents, Joseph Priestley, the enlightenment chemist and theologian, left England in 1796 due to religious persecution and settled on the Susquehanna River. His former house (originally purchased by chemists from Pennsylvania State University after a colloquium that founded the American Chemical Society) is a historical museum .