The Gotthard Base Tunnel (GBT) is a railway tunnel under construction in Switzerland. With a planned length of 57 km (35.4 mi) and a total of 153.5 km (95.4 mi) of tunnels, shafts and passages planned, it will be the longest tunnel (of all railway and road tunnels) in the world upon completion, ahead of the current record holder, the Seikan Tunnel (connecting the Japanese islands of Honshū and Hokkaidō).

Completion has been projected for 2015 but due to delays the tunnel may not be completed before 2018. With a breakthrough in one of the tunnels achieved on June 15 2009, an opening date for the tunnels during 2017 seems likely since work currently is progressing at a higher pace than projected.

The project is designed to feature two separate tunnels containing one track each. The tunnel is part of the Swiss AlpTransit project, also known as New Railway Link through the Alps NRLA which also includes the Lötschberg Base Tunnel between the cantons of Berne and Valais. Like the Lötschberg tunnel, it is intended to bypass winding mountain routes and establish a direct route suitable for high speed rail and heavy freight trains. On completion it is expected to decrease the current 3.5 hours travel time from Zürich to Milan by one hour. It will also reduce the time between Zürich and Lugano to 1 hour 40 minutes. The two portals will be near the villages of Erstfeld, Canton Uri and Bodio, Canton Ticino.

Nearby are two more St. Gotthard Tunnels: the 1881 Gotthard Rail Tunnel and the 1980 Gotthard Road Tunnel.