The Maunsell Forts are towers built in the Thames and Mersey estuaries during the Second World War to help defend the United Kingdom. They were operated as army and navy forts, and named after their designer, Guy Maunsell. The forts were decommissioned during the late 1950s and later used for other activities including pirate radio broadcasting. One of the forts is managed by the unrecognised Principality of Sealand; boats visit the remaining forts occasionally, and a consortium named Project Redsands is planning to conserve the fort situated at Red Sands. The aesthetic attraction of the Maunsell forts has been considered to be associated with the aesthetics of decay, transience and nostalgia.

During the summers of 2007 and 2008, Red Sands Radio, a station commemorating the pirate radio stations of the 1960s, operated from the Red Sands fort on 28-day Restricted Service Licences. The fort was subsequently declared unsafe, and Red Sands Radio has moved its operations ashore to Whitstable.

Forts had been built in river mouths and similar locations to defend against ships, such as the Grain Tower Battery at the mouth of the Medway dating from 1855, Plymouth Breakwater Fort, completed 1865, the four Spithead Forts: Horse Sand Fort, No Mans Land and St Helens Forts which were built 1865–1880 and Spitbank Fort, built during the 1880s, the Humber Forts on Bull & Haile Sands, completed in late 1919, and the Nab Tower, intended as part of a World War I anti-submarine defence but only set in place in 1920.

The Maunsell naval forts were built in the Thames estuary and operated by the Royal Navy, to deter and report German air raids following the Thames as a landmark, and prevent attempts to lay mines by aircraft in this important shipping channel. There were four naval forts:

This artificial naval installation is similar in some respects to early "fixed" offshore oil platforms. It consisted of a rectangular 51.2 metres (168 ft) by 21.2 metres (70 ft) reinforced concrete pontoon base with a support superstructure of two 60-foot (18 m) tall, 24-foot (7.3 m) diameter hollow reinforced concrete towers, walls roughly 3.5 inches (9 cm) thick; overall weight is estimated to have been approximately 4,500 tons. The twin concrete supporting towers were divided into seven floors, four for crew quarters; the remainder provided dining, operational, and storage areas for several generators, and for fresh water tanks and antiaircraft munitions. There was a steel framework at one end supporting a landing jetty and a crane, which was used to hoist supplies aboard; the wooden landing stage itself became known as a "dolphin".

The towers were joined above the eventual waterline by a steel platform deck upon which other structures could be added; this became a gun deck, on which an upper deck and a central tower unit were constructed. QF 3.7 inch anti-aircraft guns were positioned at each end of this main deck, with a further two Bofors 40 mm anti-aircraft guns and the central tower radar installations atop a central living area that contained a galley, medical, and officers quarters.

The design of these concrete structures is equal to a military grade bunker, due to the ends of the stilts (under water), that are locked into the ground. Many species of fish live near the forts because the forts create cover. They have provided landmark references for shipping. They were laid down in dry dock and assembled as complete units. They were then fitted out—the crews going aboard at the same time for familiarization—before being towed out and sunk onto their sand bank positions in 1942.

The naval fort design was the latest of several that Maunsell had devised in response to Admiralty inquiries. Early ideas had considered forts in the English Channel able to combat enemy vessels.