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 (21 km) north of Maryland and the Mason-Dixon line and 52 miles (84 km) southwest of Harrisburg, the state capital. According to the United States Census Bureau, Chambersburg's 2020 population was 21,903. When combined with the surrounding Greene, Hamilton, and Guilford Townships, the population of Greater Chambersburg is 52,273 people. The Chambersburg, PA Metropolitan Statistical Area includes surrounding Franklin County, and in 2010 included 149,618 people.

Franklin County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 155,932. Its county seat is Chambersburg.