Bloomsburg–Berwick metropolitan area in the context of Columbia County, Pennsylvania

Northumberland County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 91,647. Its county seat is Sunbury. The county is part of the Central region of the commonwealth.

The county was formed in 1772 from parts of Lancaster, Berks, Bedford, Cumberland, and Northampton Counties and named for the county of Northumberland in northern England. Northumberland County is a fifth class county according to the Pennsylvania's County Code. Northumberland County comprises the Sunbury, Pennsylvania Micropolitan Statistical Area, which is also included in the Bloomsburg-Berwick-Sunbury, PA Combined Statistical Area. Among its notable residents are Thomas L. Hamer, a Democratic member of Congress in the 1830s, and Joseph Priestley, the Enlightenment chemist and theologian, who left England in 1796 due to religious and political 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.

Sunbury (/ˈsʌnbɛri/ SUN-berr-ee) is a city and the county seat of Northumberland County in Pennsylvania, United States. Located in the Susquehanna Valley, Sunbury is positioned on the east bank of the Susquehanna River.

Sunbury's roots stretch back to the early 18th century. As of the 2010 census, it had a population of 9,905. The city is one of the three principal cities in the larger Bloomsburg–Berwick metropolitan area.