Duncannon is a borough in Perry County, Pennsylvania, United States. The population was 1,508 at the 2000 census. It is part of the Harrisburg–Carlisle Metropolitan Statistical Area. The center of population of Pennsylvania is located in Duncannon. It is named after the coastal town of Duncannon, County Wexford, Ireland. Lightning Guider sleds were manufactured in Duncannon from 1904 until 1988. The Appalachian Trail runs along Market Street, the main street in Duncannon.
Duncannon is located at 40°23′38″N 77°1′44″W / 40.39389°N 77.02889°W / 40.39389; -77.02889 (40.393986, -77.028891).
According to the United States Census Bureau, the borough has a total area of 0.4 square miles (1.1 km²), all of it land.