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 Pennsylvania region of the state. 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.