Chambersburg is a borough in and the county seat of Franklin County, in the South Central region of Pennsylvania, United States. It is in the Cumberland Valley, which is part of the Great Appalachian Valley, and 13 miles north of Maryland and the Mason-Dixon line and 52 miles southwest of Harrisburg, the state capital. According to the United States Census Bureau, Chambersburg's 2020 population was 21,903.