The Blackwall Tunnel is a road tunnel underneath the River Thames in London, linking the London Borough of Greenwich with the London Borough of Tower Hamlets. It is actually two tunnels, one built at the end of the 19th century, the other completed some 70 years later.

The southern entrances are just west of the Millennium Dome; the northern portal lies just south of the A13 in Poplar.

The tunnel crossing is a key link between north and south sides of the river and forms part of a key route into central London from south-east London and Kent. The southern approach takes traffic from the A2 trunk route, turning into a motorway (A102M) for two miles, before reverting to a trunk route again a few hundred metres short of the tunnel entrance.

The older western tunnel, designed by Sir Alexander Binnie and built by S Pearson & Sons for the London City Council at a cost of 1.4 million, was opened by the Prince of Wales on 22 May 1897 and was then the longest underwater tunnel in the world (4,410 feet long). It took six years to construct, using tunnelling shield and compressed air techniques (shield pioneer James Henry Greathead was a consultant; Sir Joseph Bazalgette was also involved in the original planning of the project), and seven people died in the process. To clear the site in Greenwich, more than 600 houses had to be demolished, including one reputedly once owned by Sir Walter Raleigh.

Today the western bore is only used for north-bound traffic (and not accessible to high-sided vehicles), features a striking gateway built of red-brick. The tunnel itself has several sharp bends. Some suggest these were built so that horses (motor vehicles were very rare back in 1897!) wouldn't bolt once they saw the daylight. The tunnel is just about wide enough to carry two lanes of traffic, though larger vehicles need to keep to the left-hand high lane so that they don't hit the roof!

The newer tunnel, opened on 2 August 1967, is much wider, with fewer height restrictions and no sharp corners - very much designed for 20th century road traffic.

The tunnel is a notorious traffic bottleneck, with long tailbacks in the morning rush-hour on weekdays as traffic heads north from SE London and Kent towards central London. The transport authorities try to increase flows by opening one lane of the eastern tunnel to northbound traffic for two hours; this, of course, means there is only one lane for southbound traffic so the northern approaches quickly become jammed each morning. Unfortunately, the western tunnel is not suitable to operate a similar two-way flow in the evening, so the northern approaches are frequently even more jammed in the late afternoon/early evening.

The nearest alternative crossings are: to the west, the Rotherhithe Tunnel (about three miles closer to central London) and Tower Bridge, or, to the east, the Woolwich Ferry (about two miles to the east) or the Dartford Crossing (some 15 miles away, carrying the M25 northbound under the river by tunnel and southbound over the river via the Queen Elizabeth II Bridge).

The tunnels do not allow pedestrians, but a bus route (number 108) operates through the tunnel. When the service is not running, the nearest pedestrian crossings of the river are the Woolwich foot tunnel adjacent to the Woolwich Ferry (see above) and the Greenwich foot tunnel about two miles to the west.