The Gotthard Base Tunnel (GBT) is a railway tunnel in the heart of the Swiss Alps expected to open in 2016. With a route length of 57 km (35.4 mi) and a total of 151.84 km (94.3 mi) of tunnels, shafts and passages, it is the world's longest rail tunnel, surpassing the Seikan Tunnel in Japan.

Its main purpose is to increase total transport capacity across the Alps, especially for freight, notably between Germany and Italy, and more particularly to shift freight volumes from road to rail to reduce environmental damage caused by ever-increasing numbers of heavy lorries. A secondary benefit will be to cut the journey time for passenger trains from Zürich to Milan by about an hour and from Zürich to Lugano to 1-hour 40 minutes. The project consists of two single-track tunnels. It is part of the AlpTransit project, also known as the New Railway Link through the Alps (NRLA), which includes the Lötschberg Base Tunnel between the cantons of Bern and Valais and the under construction Ceneri Base Tunnel (scheduled to open late 2019) to the south. It bypasses the Gotthardbahn, a winding mountain route opened in 1882 across the Saint-Gotthard Massif, which is now operating at capacity, and establishes a direct route usable by high-speed rail and heavy freight trains.
The total cost of the project is 9.8 billion Swiss francs, or US$10.3 billion. When completed, the Gotthard Base Tunnel will be one of the longest tunnel construction projects in the world: 20 years of constant construction and preparation.