The Simplon Tunnel is a railway tunnel on the Simplon railway that connects Brig, Switzerland and Domodossola, Italy, through the Alps, providing a shortcut under the Simplon Pass route. It is straight except for short curves at either end. It consists of two single-track tunnels built nearly 15 years apart. The first to be opened is 19,803 m (64,970 ft) long; the second is 19,824 m (65,039 ft) long, making it the longest railway tunnel in the world for most of the twentieth century, from 1906 until 1982, when the Daishimizu Tunnel opened.

The Simplon Railway is a line that links Lausanne in Switzerland and Domodossola in Italy, via Brig. The 20 km (12 mi)-long Simplon Tunnel is a major part of it. The line between Lausanne and Vallorbe is sometimes considered to form part of the line, making it 233 km (145 mi) long.