Lewisburg is a borough in Union County, Pennsylvania, United States, 30 miles south by southeast of Williamsport and 60 miles north of Harrisburg. The population was 5,158 as of the 2020 census. It is the county seat of Union County. Located in central Pennsylvania's Susquehanna Valley, on the West Branch Susquehanna River, it is home to Bucknell University. Its 19th-century downtown is listed on the National Register of Historic Places.