Jefferson County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 45,200. 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 groundhog that predicts when spring will come every February 2 (Groundhog Day).