The Sydney Harbour Tunnel is a twin tube road tunnel in Sydney, Australia. The tunnel was completed and opened to traffic in August 1992 to provide a second vehicular crossing of Sydney Harbour to alleviate congestion on the Sydney Harbour Bridge.

The tunnel joins the Warringah Freeway at North Sydney, and the Cahill Expressway at the entrance to the Domain Tunnel. It has two lanes in each direction, and runs at an angle of approximately thirty degrees (North to South) to the Sydney Harbour Bridge, which has eight lanes, with a tidal flow operation. In 2005, the tunnel was carrying around 86,800 vehicles per day.