Operation Grapple was a set of four series of British nuclear weapons tests of early atomic bombs and hydrogen bombs carried out in 1957 and 1958 at Malden Island and Kiritimati (Christmas Island) in the Gilbert and Ellice Islands in the Pacific Ocean (modern Kiribati) as part of the British hydrogen bomb programme. Nine nuclear explosions were initiated, culminating in the United Kingdom becoming the third recognised possessor of thermonuclear weapons, and the restoration of the nuclear Special Relationship with the United States in the form of the 1958 US–UK Mutual Defence Agreement.

During the Second World War, Britain had a nuclear weapons project, codenamed Tube Alloys, which was merged with the American Manhattan Project in August 1943. Many of Britain's top scientists participated in the Manhattan Project. After the war, fearing that Britain would lose its great power status, the British government resumed the atomic bomb development effort, now codenamed High Explosive Research.

The successful test of an atomic bomb in Operation Hurricane in October 1952 represented an extraordinary scientific and technological achievement, but Britain was still several years behind the United States, which had developed the more powerful thermonuclear weapons in the meantime. In July 1954, the Cabinet agreed that the maintenance of great power status required that Britain also develop thermonuclear weapons.

The United Kingdom Atomic Energy Authority Atomic Weapons Research Establishment at Aldermaston produced three designs: Orange Herald, a large boosted fission weapon; Green Bamboo, an interim thermonuclear design; and Green Granite, a true thermonuclear weapon. The new designs had to be tested to demonstrate that they worked, hence Operation Grapple. The first series consisted of three tests in May and June 1957. In the first test, Grapple 1, a version of Green Granite known as Short Granite was dropped from a Vickers Valiant bomber flown by Wing Commander Kenneth Hubbard. The bomb's yield was estimated at 300 kilotonnes of TNT (1,260 TJ), far below its designed capability. Despite this, the test was hailed as a successful thermonuclear explosion, and the government did not confirm or deny reports that the UK had become a third thermonuclear power. The second test, Grapple 2, was of Orange Herald; its 720-to-800-kilotonne-of-TNT (3,010 to 3,350 TJ) yield made it technically a megaton-range weapon, and the largest ever achieved by a single stage nuclear device. Grapple 3 tested Purple Granite, a version of Short Granite with some fixes; its yield was only 200 kilotonnes of TNT (837 TJ).

A second test series was required, which consisted of a single test, Grapple X, in November 1957. This time the yield of 1.8 megatonnes of TNT (7.53 PJ) exceeded expectations. It was a true hydrogen bomb, but most of its yield came from nuclear fission rather than nuclear fusion. In a third series with a single test, Grapple Y, in April 1958, another design was trialled. With an explosive yield of about 3 megatonnes of TNT (12.6 PJ), it remains the largest British nuclear weapon ever tested. The design of Grapple Y was notably successful because much of its yield came from its thermonuclear fusion reaction instead of fission of a heavy uranium-238 tamper—the dense material surrounding the core that kept the reacting mass together to increase its efficiency. Its yield had been closely predicted, indicating that its designers understood the process. A final series of four tests in August and September 1958, known as Grapple Z, tested techniques for boosting and making bombs immune to predetonation caused by nearby nuclear explosions. Two of these tests were detonations from balloons. A moratorium on testing came into effect in October 1958, and Britain never resumed atmospheric nuclear testing.

During the early part of the Second World War, Britain had a nuclear weapons project, codenamed Tube Alloys. At the Quebec Conference in August 1943, Prime Minister Winston Churchill and US President Franklin Roosevelt signed the Quebec Agreement, which merged Tube Alloys with the American Manhattan Project to create a combined British, American and Canadian project. The September 1944 Hyde Park Aide-Mémoire extended both commercial and military cooperation into the post-war period. Many of Britain's top scientists participated in the Manhattan Project.

The British government had trusted that America would continue to share nuclear technology, which it considered to be a joint discovery. On 16 November 1945, Churchill and Roosevelt's successors, Clement Attlee and Harry S. Truman, signed a new agreement that replaced the Quebec Agreement's requirement for "mutual consent" before using nuclear weapons with one for "prior consultation", and there was to be "full and effective cooperation in the field of atomic energy", but this was only "in the field of basic scientific research". The United States Atomic Energy Act of 1946 (McMahon Act) ended technical cooperation. The revelation of a Canadian spy ring that included British physicist Alan Nunn May while the bill was being prepared caused the United States Congress to add the death penalty for sharing "restricted data" with foreign nations. Efforts to restore the nuclear Special Relationship with the United States over the following decade were dogged by repeated spy scandals, including the arrest of Klaus Fuchs in 1950, and the defection of Guy Burgess and Donald Maclean in 1951. Fearing a resurgence of American isolationism and Britain losing its great power status, the British government restarted its own development effort, now codenamed High Explosive Research.

The successful test of an atomic bomb in Operation Hurricane in October 1952 represented an extraordinary scientific and technological achievement. Britain became the world's third nuclear power, reaffirming the country's status as a great power, but hopes that the United States would be sufficiently impressed to restore the Special Relationship were soon dashed. In November 1952, the United States conducted Ivy Mike, the first successful test of a true thermonuclear device or hydrogen bomb, a far more powerful form of nuclear weapons. Britain was therefore still several years behind in nuclear weapons technology. The Defence Policy Committee, chaired by Churchill and consisting of the senior Cabinet members, considered the political and strategic implications in June 1954, and concluded that "we must maintain and strengthen our position as a world power so that Her Majesty's Government can exercise a powerful influence in the counsels of the world." A Cabinet meeting on 27 July accepted this argument, and directed the Lord President to proceed with the development of thermonuclear weapons.