Jefferson County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 44,492. Its county seat is Brookville. The county was established on March 26, 1804, from part of Lycoming County and later organized in 1830. It is named for President Thomas Jefferson. It is home to Punxsutawney Phil, the most famous groundhog that predicts when spring will come every February 2 on Groundhog Day. The county is part of the North Central Pennsylvania region of the state.

Brookville is a borough in Jefferson County in the U.S. state of Pennsylvania, 70 miles (110 km) northeast of Pittsburgh. As of the 2010 census, the population was 3,933. Founded in 1830, it is the county seat of Jefferson County.