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Ivy Mike was the codename given to the first full-yield test of a multi-stage thermonuclear device, also known as the Teller–Ulam design. Ivy Mike was detonated on November 1, 1952, by the United States on the island of Elugelab in Enewetak Atoll, Pacific Proving Grounds, as part of Operation Ivy. Yielding 10.4 megatons of TNT, it dwarfed the previous largest test Greenhouse George (250 kilotons, 1951), and remains the fourth most powerful nuclear test by the US and tenth most powerful overall. The test was a key point in the nuclear arms race between the US and Soviet Union during the Cold War.

US President Harry S. Truman ordered a full-scale hydrogen bomb program in January 1950, five months after the first Soviet nuclear test RDS-1. By March 1951, physicists Edward Teller and Stanisław Ulam produced a report detailing a two-stage design, where the X-rays from a fission bomb are directed toward radiation implosion of a secondary device containing fusion fuel. The experimental Greenhouse George test in May produced fusion with this design; Teller remarked "Eniwetok would not be large enough for the next one".

The device, nicknamed Sausage, was more than 20 ft (6.1 m) tall, and a proof of concept, unsuitable for use as a deliverable weapon. It was designed and constructed at Los Alamos Scientific Laboratory, while design calculations led by John Archibald Wheeler used the UNIVAC I computer. The device consisted of a fission-based Mark 5 nuclear bomb acting as the nuclear primary, which compressed the cylindrical secondary fusion device. This was fuelled by 1,000 liters of cryogenic liquid deuterium in a vacuum flask. A plutonium sparkplug rod in the flask's center aided fusion ignition in the secondary, while the majority of the device yield came from the natural uranium tamper of the secondary, which underwent fast fission by fusion neutrons.

Samples from the explosion led to the discovery of the predicted elements with atomic number 99 and 100, later named einsteinium and fermium. They also contained traces of the isotopes plutonium-246, and plutonium-244. These new nuclei had been produced by the rapid neutron capture process, later formalized in nuclear astrophysics in 1957. The Soviet Union tested a single-stage thermonuclear design, RDS-6s, in August 1953, and a three-megaton two-stage design, RDS-37, in 1955. The United Kingdom (in 1957), China (in 1967), and France (in 1968) later tested multi-megaton thermonuclear devices in the atmosphere.