The Seikan Tunnel (青函トンネル Seikan Tonneru) is currently the longest railway tunnel in the world, at 53.9 km (33.4 miles), and will be until the new Gotthard Base Tunnel with 57 km (36 miles) is opened. The Seikan Tunnel is slightly longer than the Channel Tunnel and contains a 23.3 km portion under the seabed. It connects the Japanese islands of Honshu and Hokkaido, as part of the Japan Railway Kaikyo Line.

The tunnel was completed in 1988. The volcanic rock beneath the Tsugaru Strait was too unstable for boring, so engineers had to blast most of the tunnel out with dynamite.

Each of the twin tracks inside the tunnel is built with three rails, so that both narrow-gauge and standard-gauge trains can be handled. Currently, only narrow-gauge trains use the tunnel: there are long-term plans to link the tunnel into the Shinkansen network, although these are not expected to be completed until after 2020.

There are two passenger stations inside the tunnel (Yoshioka-Kaitei Station and Tappi-Kaitei Station ), both of which are museums detailing the history and function of the tunnel.