North portal of the Fréjus Rail Tunnel under the Alps

The Fréjus Rail Tunnel (also called Mont Cenis Tunnel) is a railroad tunnel of 13.7 km (8.5 miles) length in the European Alps, carrying the Fréjus railway through Mount Cenis and connecting Modane, France and Bardonecchia, Italy. It passes beneath the Pointe du Fréjus (2932 m) and the Col de Fréjus (2542 m).

The initial gallery was 12.8 kilometers long, twice as much as the previously longest tunnel. Drilling started in August 1857 from Bardonecchia and in December 1857 from Modane. On 26 December 1870, French and Italian workers shook hands as both teams met halfway: the galleries were perfectly aligned to about 40 cm horizontally and 60 cm vertically. The tunnel opened for traffic on 17 September 1871, thus making it the oldest of the large tunnels through the Alps. The gallery was extended to its present length in 1881 with a new reinforced entrance on the French side.

The construction, directed by Germain Sommeiller, was scheduled to take 25 years, but was completed in only 14 years thanks to technical innovations such as pneumatic drilling machines and electrical ignition of explosive charges. In the final construction years, the use of recently-invented dynamite further accelerated the tunnel's completion. The next two Alpine tunnels were built with similar techniques: the Gotthard Rail Tunnel opened in 1882 and the Simplon Tunnel in 1906.

From 1868 to 1871, there was already a railroad line in operation, going over the pass; it was primarily used to transport English mail to India.

Today the Fréjus tunnel remains an important link in the connection between Rome and Paris, via Turin and Chambéry. Following the development of car and truck transportation, the Fréjus Road Tunnel was built along the same path from 1974 to 1980. A future high-speed rail tunnel to improve transit capacity between France and Italy is being planned as part of the Lyon Turin Ferroviaire initiative.