The gate in the middle of this view has been raised to the underspill position and a maintenance barge is in attendance.

Built across a 523 metre wide stretch of the river, the barrier divides the river into six navigable and four smaller non-navigable channels between nine large concrete piers. The flood gates across the openings are radial, i.e., half-cylindrical, and they operate by rotating, raised by hydraulics out of a horizontal sill below the water to form the barrier. They can rotate further to allow for 'underspill' for maintenance. All the gates are made of steel. The four large central gates are 61 metres long, 10.5 metres high (above local ground level) and weigh 1,500 tonnes; the outer two gates are 31.5 metres. Additionally, four radial gates by the riverbanks can be lowered. These gate openings, unlike the main six, are non-navigable.

London is quite vulnerable to flooding. The threat has increased over time due to the slow but continuous rise in high water level over the centuries (75 cm / 100 years) and the slow 'tilting' of Britain (up in the North and down in the South) caused by post-glacial rebound. This general rise in potential water levels combined with the tidal conditions of the Thames and with particularly severe weather conditions can create serious flood conditions — surge tides. After 307 people died in the UK in the North Sea Flood of 1953 the issue gained new prominence. Early proposals for a flood control system were stymied by the need for a large opening in the barrier to allow for vessels from London Docks to pass through. When containerization came in and a new port was opened at Tilbury, a smaller barrier became feasible with each of the four main navigation spans being the same width as the span of Tower Bridge.

The concept of the rotating gates was devised by Charles Draper. The barrier was designed by Rendel, Palmer and Tritton for the GLC. The site at Woolwich was chosen because of the relative straightness of the banks, and because the underlying river rock was strong enough to support the barrier. Work began at the barrier site in 1974 and construction was largely complete by 1982. In addition to the barrier itself the flood defences for 11 miles down river were raised and strengthened. The barrier was officially opened on May 8, 1984. Total construction cost was around £534 m (£1.3 billion in 2001 pounds) with an additional £100 m for river defences. The barrier was designed to cope with sea level rises until around 2030–2050. Based on current estimates, it is expected to serve its full term. Since 1982 the barrier has been raised over 90 times; further, it is raised every month for testing. The barrier was operated by the National Rivers Authority until April 1996 when it passed to the Environment Agency.

In 2005, a suggestion that it might become necessary to supersede the Thames Barrier with a much more ambitious ten-mile-long barrier across the Thames Estuary from Sheerness in Kent to Southend in Essex was made public.