The Flat Rock Tunnel is an active railroad tunnel that is part of Norfolk Southern's Harrisburg Line near Manayunk, Pennsylvania, United States. The tunnel was built by the Reading Railroad as part of its railroad line along the Schuylkill River.

Construction of the tunnel began in 1836, and it opened in 1840. Between 1858 and 1859, the Flat Rock and Black Rock Tunnels were widened to allow for larger trains from the Lebanon Valley Branch. The distance between the tracks was increased from 4 feet (1.2 m) to 6 feet (1.8 m). This project was the first to use electric detonation to trigger multiple explosive charges.