The Kanmon Straits (関門海峡 Kanmon Kaikyō) or Straits of Shimonoseki is the stretch of water separating two of Japan's four main islands. On the Honshu side of the water is Shimonoseki (下関, which contributed "Kan" to the name of the strait) and on the Kyushu side is Kitakyushu, whose former city and present ward, Moji (門司), gave the strait its "mon".

There are The Major (Ōseto) and The Minor (Koseto, or Odo inlet). Generally speaking, The Kanmon Straits means The Major.

The Kanmon Straits can be traversed in a number of ways, the oldest of which is the ferry. There is also a bridge which carries an expressway. By far the most used method is a number of tunnels which carry the Shinkansen, trains, cars and even pedestrians. The first railway tunnel was opened on November 15, 1942. The road tunnel was opened on March 9, 1958. The Kanmonkyo bridge (see photo) was opened to vehicles on November 14, 1973. The Shinkansen tunnel was opened on March 10, 1975.