A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission (fission or atomic bomb) or a combination of fission and nuclear fusion reactions (thermonuclear weapon), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.

Nuclear weapons have had yields between 10 tons (the W54) and 50 megatons for the Tsar Bomba (see TNT equivalent). Yields in the low kilotons can devastate cities. A thermonuclear weapon weighing as little as 600 pounds (270 kg) can release energy equal to more than 1.2 megatons of TNT (5.0 PJ). Apart from the blast, effects of nuclear weapons include extreme heat and ionizing radiation, firestorms, radioactive nuclear fallout, an electromagnetic pulse, and a radar blackout.

The first nuclear weapons were developed by the United States in collaboration with the United Kingdom and Canada during World War II in the Manhattan Project. Production requires a large scientific and industrial complex, primarily for the production of fissile material, either from nuclear reactors with reprocessing plants or from uranium enrichment facilities. Nuclear weapons have been used twice in war, in the 1945 atomic bombings of Hiroshima and Nagasaki that killed between 150,000 and 246,000 people. Nuclear deterrence, including mutually assured destruction, aims to prevent nuclear warfare via the threat of unacceptable damage and the danger of escalation to nuclear holocaust. A nuclear arms race for weapons and their delivery systems was a defining component of the Cold War.

Strategic nuclear weapons are targeted against civilian, industrial, and military infrastructure, while tactical nuclear weapons are intended for battlefield use. Strategic weapons led to the development of dedicated intercontinental ballistic missiles, submarine-launched ballistic missile, and nuclear strategic bombers, collectively known as the nuclear triad. Tactical weapons options have included shorter-range ground-, air-, and sea-launched missiles, nuclear artillery, atomic demolition munitions, nuclear torpedos, and nuclear depth charges, but they have become less salient since the end of the Cold War.

As of 2025^[update], there are nine countries on the list of states with nuclear weapons, and six more agree to nuclear sharing. Nuclear weapons are weapons of mass destruction, and their control is a focus of international security through measures to prevent nuclear proliferation, arms control, or nuclear disarmament. The total from all stockpiles peaked at over 64,000 weapons in 1986, and is around 9,600 today. Key international agreements and organizations include the Treaty on the Non-Proliferation of Nuclear Weapons, the Comprehensive Nuclear-Test-Ban Treaty and Comprehensive Nuclear-Test-Ban Treaty Organization, the International Atomic Energy Agency, the Treaty on the Prohibition of Nuclear Weapons, and nuclear-weapon-free zones.

Nuclear weapons have only twice been used in warfare, both times by the United States against Japan at the end of World War II. On August 6, 1945, the United States Army Air Forces (USAAF) detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the USAAF detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki. These bombings caused injuries that resulted in the deaths of approximately 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are to this day, still subjects of debate.

Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over 2,000 times for testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are (chronologically by date of first test) the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, China, India, Pakistan, and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium, the Netherlands, and Belarus are nuclear weapons sharing states. South Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons.

There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, and those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output.