The Cenotaph is a war memorial on Whitehall in London, England. Designed by Sir Edwin Lutyens, it was unveiled in 1920 as the United Kingdom's national memorial to the dead of Britain and the British Empire of the First World War, was rededicated in 1946 to include those of the Second World War, and has since come to represent the Commonwealth casualties from those and subsequent conflicts. The word cenotaph is derived from Greek, meaning 'empty tomb'. Most of the dead were buried close to where they fell; thus, the Cenotaph symbolises their absence and is a focal point for public mourning. The original temporary Cenotaph was erected in 1919 for a parade celebrating the end of the First World War, at which more than 15,000 servicemen, including French and American soldiers, saluted the monument. More than a million people visited the site within a week of the parade.

Calls for the Cenotaph to be rebuilt in permanent form began almost immediately. After some debate, the government agreed and construction work began in May 1920. Lutyens added entasis (curvature) but otherwise made minimal design alterations. The Cenotaph is built from Portland stone. It takes the form of a tomb chest atop a rectangular pylon, which diminishes as it rises. Three flags hang from each of the long sides. The memorial is austere, containing almost no decoration. The permanent Cenotaph was unveiled by King George V on 11 November 1920 in a ceremony combined with the repatriation of the Unknown Warrior, an unidentified British serviceman to be interred in Westminster Abbey. After the unveiling, millions more people visited the Cenotaph and the Unknown Warrior. The memorial met with public acclaim and has largely been praised by academics, though some Christian organisations disapproved of its lack of overt religious symbolism.

The Cenotaph has been revered since its unveiling, and while nationally important has been the scene of several political protests and vandalised with spray paint twice in the 21st century. The National Service of Remembrance is held annually at the site on Remembrance Sunday; it is also the scene of other remembrance services. The Cenotaph is a Grade I listed building and forms part of a national collection of Lutyens's war memorials. Dozens of replicas were built in Britain and other Commonwealth countries. While there was no set or agreed standard for First World War memorials, the Cenotaph proved to be one of the most influential models for such structures. Lutyens designed several other cenotaphs, which all shared common features with that at Whitehall. The Cenotaph has been the subject of several artworks and has featured in multiple works of literature, including a novel and several poems. The public acclaim for the monument was responsible for Lutyens becoming a national figure, and the Royal Institute of British Architects awarded him its Royal Gold Medal in 1921. For several years afterwards much of his time was taken up with war memorial commissions.

The First World War produced casualties on a scale previously unseen by developed nations. More than 1.1 million men from the British Empire were killed. In the war's aftermath, thousands of war memorials were erected across Britain and its empire, and on the former battlefields. Amongst the most prominent designers of war memorials was Sir Edwin Lutyens, described by Historic England as "the foremost architect of his day". Lutyens established his reputation designing country houses for wealthy clients around the turn of the 20th century; his first major public commission was the design of much of New Delhi, the new capital of British India. The war had a profound effect on Lutyens and following it he devoted much of his time to the commemoration of its casualties. By the time he was commissioned for the Cenotaph, he was already acting as an adviser to the Imperial War Graves Commission (IWGC). In 1917, he travelled to France under the auspices of the IWGC and was horrified by the scale of destruction. The experience influenced his later designs for war memorials and led him to the conclusion that a different form of architecture was required to properly memorialise the dead. He felt that neither realism nor expressionism could adequately capture the atmosphere at the end of the war.

Lutyens's first war memorial was the Rand Regiments Memorial (1911) in Johannesburg, South Africa, dedicated to casualties of the Second Boer War (1899–1902). His first commission for a memorial to the First World War was for Southampton Cenotaph. The word cenotaph derives from the Greek word kenotaphion, meaning 'empty tomb'. Lutyens first encountered the term in connection with Munstead Wood, in Busbridge, Surrey. There he designed a house for Gertrude Jekyll in the 1890s—one of his earliest major commissions. In the garden was a seat in the form of a rectangular block of elm set on stone, which Charles Liddell—a friend of Lutyens and Jekyll and a librarian at the British Museum—christened the "Cenotaph of Sigismunda".

From 1915, the British government prohibited the repatriation of the bodies of men killed overseas, meaning that most bereaved families did not have a nearby grave to visit and thus war memorials became a focal point for their grief. Cenotaphs originated in Ancient Greek tradition, where they were built when it was impossible to recover a body after the battle, as the Greeks placed great cultural importance on the proper burial of their war dead. Lutyens remembered the term when working on Southampton's memorial in early 1919. He broke with the Ancient Greek convention in that his designs for London's and Southampton's cenotaphs contained no explicit reference to battle. The result at Southampton (unveiled a week before London's permanent Cenotaph) lacks the subtlety of London's monument, but introduces several design elements common in Lutyens's subsequent memorials.

The First World War ended with the Armistice of 11 November 1918, although it was not officially declared over until the signing of the Treaty of Versailles on 28 June 1919. The British government planned to hold a victory parade in London on 19 July, including soldiers marching to Whitehall, the centre of the British government. The initial design for what would become the Cenotaph was one of a number of temporary structures erected along the parade's route. The prime minister, David Lloyd George, learnt that the French plans for a similar parade in Paris included a saluting point for the marching troops and was keen to replicate the idea for the British parade. How Lutyens became involved is unclear, but he was close friends with Sir Alfred Mond and Sir Lionel Earle (respectively the government minister and senior civil servant at the Office of Works, which was responsible for public building projects) and it seems likely that one or both men discussed the idea with Lutyens. Lloyd George summoned Lutyens and asked him to design a "catafalque" as the centre point for the parade. Lloyd George emphasised that the structure was to be non-denominational. Lutyens met with Sir Frank Baines, chief architect at the Office of Works, the same day to sketch his idea for the Cenotaph and sketched it again for his friend Lady Sackville over dinner that night. Both sketches show the Cenotaph almost as-built.

At the end of the war, there was considerable social upheaval and civil unrest in Britain and Ireland, and industrial relations were tense. The government, fearful that revolutionary ideologies such as Bolshevism might start to take hold, hoped the parade and a central saluting point would unite the nation in celebrating the victorious conclusion to the war and commemorating the sacrifice of the dead.