The Marmaray Tunnel (Turkish: Marmaray Tüneli) is a 13.5 km (8.4 mi) long undersea railway tunnel in Istanbul, Turkey, beneath the Bosporus strait, linking Kazlıçeşme, Zeytinburnu in Europe with Ayrılıkçeşmesi in Asia. The tunnel consists of two single track tunnels with three underground railway stations: Yenikapı, Sirkeci and Üsküdar.
The Marmaray Tunnel was opened to passenger traffic on 29 October 2013. In March 2019 the overground part of the Marmaray project were completed and normal train traffic, including commuter (entire line), YHT and freight services started to run through the tunnel. The tunnel is the deepest immersed tube tunnel in the world.