Big Run is a borough in Jefferson County, Pennsylvania, United States. The population was 624 at the 2010 census, a decline from the figure of 686 tabulated in 2000. The borough was named for the waterway of the same name.

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.