The Thames Tunnel is an under-river tunnel passing under the River Thames in London. It is approximately 11 meters wide, approximately 6 meters high and approximately 396 meters long.

Built between 1825 and 1843 using the tunnel boring technique invented shortly before by Count Thomas Cochrane and Marc Isambart Brunel, it is the first known tunnel to have been built under a navigable river1.

First intended for horse-drawn traffic, it was transformed into a railway tunnel to be integrated in 1869 into the East London Railway, component of the London Underground from 1933, then of the London Overground since 2011.

It should not be confused with the Rotherhithe tunnel, a road structure located approximately 500 meters downstream.