The Shin-Kanmon Tunnel (新関門トンネル, shin-kanmon tonneru) is a 18.713-kilometre (11.628 mi) long undersea railway tunnel under the Kanmon Straits connecting Shin-Shimonoseki Station and Kokura Station.

It is also the longest tunnel section on the entire Sanyo/Tokaido Shinkansen line.

The construction of the new tunnel started as a project to connect Honshu and Kyushu with a high-speed railway.

The tunnel was constructed at the shortest section in the Kanmon Strait between Dannoura, Shimonoseki City - Mekari, Mojiku, Kitakyushu.

Although the entire Shin-Kanmon Tunnel is 5 times longer, the undersea section is shorter than the conventional line Kanmon Tunnel (the Kanmon Tunnel undersea bed section is 1,140-metre (1,250 yd) long).