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A thermonuclear weapon, fusion weapon or hydrogen bomb (H-bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a lower mass, or a combination of these benefits. Characteristics of nuclear fusion reactions make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material. The first full-scale thermonuclear test (Ivy Mike) was carried out by the United States in 1952, and the concept has since been employed by most of the world's nuclear powers in the design of their weapons.

Modern fusion weapons essentially consist of two main components: a nuclear fission primary stage and a separate nuclear fusion secondary stage containing thermonuclear fuel: heavy isotopes of hydrogen (deuterium and tritium) as the pure element or in modern weapons lithium-6 deuteride. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs.

The weapon firing begins with the detonation of the fission primary stage. Its temperature soars past 100 million kelvin, emitting X-ray bremsstrahlung. These flood the radiation case, allowing compression of the separately located secondary. A neutron shield blocks the predetonation of the secondary, which completes detonation before destruction by the primary's fireball.

The secondary stage consists of the outer pusher/tamper, fusion fuel, and central fissile sparkplug. The primary's X-rays intensely ionize and ablate the tamper surface, imploding the secondary. This triggers a fission explosion in the sparkplug. These forces combine to begin fusion ignition in the fusion fuel, around 300 million kelvin. The Jetter cycle produces the crucial tritium fuel from neutron-lithium-6 reactions.

Additionally, most weapons use a depleted uranium tamper and case. This undergoes fast fission from fast fusion neutrons and is the main contribution to the total yield and radioactive fission product fallout.

Before Ivy Mike, the US Operation Greenhouse in 1951 was the first nuclear test series investigating thermonuclear principles. Shot George tested radiation implosion and the first fusion ignition while Shot Item tested the simpler principle of a boosted fission weapon. The design of all modern thermonuclear weapons in the United States is known as the Teller–Ulam configuration for its two chief contributors, Edward Teller and Stanisław Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of physicist John von Neumann.

Multi-stage devices were independently developed by the Soviet Union, United Kingdom, France, and China. There is not enough public information to determine whether India, Israel, or North Korea possess multi-stage weapons. Pakistan is not considered to have developed them. After the collapse of the Soviet Union, Ukraine, Belarus, and Kazakhstan became the first and only countries to relinquish their thermonuclear weapons, although these had never left the operational control of Russian forces.

Thermonuclear weapons are the only artificial source of explosions above one megaton TNT. The Tsar Bomba was the most powerful bomb ever detonated at 50 megatons TNT. As they are the most efficient design for yields above 50 kilotons of TNT (210 TJ), and with decreased relevance of tactical nuclear weapons, virtually all the nuclear weapons deployed by the five recognized nuclear states today use the Teller–Ulam design. While some have been developed into clean bombs, most thermonuclear weapons designed, including all current US and UK nuclear warheads, derive most of their energy from fast fission, causing high fallout.