The Twin Peaks Tunnel is a 2-mile long subway tunnel in San Francisco, California, United States running under Twin Peaks . When it was opened on February 3, 1918, it was the longest subway tunnel in the world. It runs from the end of Market Street at Castro Street in the east to the West Portal neighborhood in the west. There are two stations along the tunnel, the Forest Hill Station, near the western end, and the now disused Eureka Station, near the eastern end.

When the Market Street Subway was built, it was connected to the Twin Peaks Tunnel to be used by the Muni Metro K Ingleside, L Taraval and M Oceanview lines. The Eureka Street Station was closed, and the metro lines stop at the nearby Castro Street Station at the western end of the Market Street Subway instead. A new station with high-platforms, the West Portal Station, was also built for the metro lines at the western end of the Twin Peaks Tunnel.