The Seikan Tunnel (青函トンネル Seikan Tonneru or 青函隧道 Seikan Zuidō) is a 53.85-kilometre (33.46 mi) railway tunnel in Japan, with a 23.3-kilometre (14.5 mi) long portion under the seabed. Track level is about 140 metres (460 ft) below seabed and 240 m (790 ft) below sea level. It travels beneath the Tsugaru Strait—connecting Aomori Prefecture on the Japanese island of Honshu and the island of Hokkaido—as part of the Kaikyo Line of Hokkaido Railway Company. The name Seikan comes from combining the on'yomi readings of the first characters of Aomori (青森^?) and Hakodate (函館^?), the nearest major city on the Hokkaido side.

Seikan is both the longest and the deepest operational rail tunnel in the world, although the Gotthard Base Tunnel in Switzerland will be longer when it opens to traffic in 2016. It is also the longest undersea tunnel in the world, although the Channel Tunnel between the United Kingdom and France has a longer undersea portion.