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.

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.

Plans to build a modern trans-Bosporus tunnel were proposed in 1997, based on a feasibility study ten years prior, and the necessary capital was secured in 1999.

The completion of the project was delayed due to archeological discoveries near Sirkeci, with artifacts dating back 8,000 years.

The western portal to the tunnel is in Kazlıçeşme, Zeytinburnu on the European side of the city, just west of Fatih.

The eastern portal to the tunnel is located in Ayrılıkçeşmesi, Kadıköy on the Asian side of the city.

[3] Before the tunnel sections were lowered, a portion of the ground underneath the Bosporus needed to be strengthened.