Nuclear terrorism is the use of a nuclear weapon or radiological weapon as an act of terrorism. There are many possible terror incidents, ranging in feasibility and scope. These include the sabotage of a nuclear facility, the intentional irradiation of citizens, or the detonation of a radiological device, colloquially termed a dirty bomb, but consensus is lacking. According to the 2005 United Nations International Convention for the Suppression of Acts of Nuclear Terrorism nuclear terrorism is an offense committed if a person unlawfully and intentionally "uses in any way radioactive material … with the intent to cause death or serious bodily injury; or with the intent to cause substantial damage to property or to the environment; or with the intent to compel a natural or legal person, an international organization or a State to do or refrain from doing an act."

The possibility of terrorist organizations using nuclear weapons has been identified by nuclear powers and considered since the conception of nuclear weapons and the rise of global terrorism. Nuclear powers collaborate to prevent terror organizations from acquiring nuclear weapons and fuel.

It is considered plausible that terrorists could acquire a nuclear weapon. As such, countries such as China and the UK have taken steps to restrict access to nuclear weapons and materials. Restrictions being implemented are only a small part of prevention systems that are being researched in countries. Preventing nuclear terrorism is a topic that interests countries who are developing or expanding their nuclear warhead knowledge. Whether it be through policy, summits, or warhead detection and disablement, the idea of threatening another nation is a worry that comes with the notion of war. Nonetheless, despite thefts and trafficking of small amounts of fissile material, there is no credible evidence that any terrorist group has ever obtained or produced nuclear materials of sufficient quantity or purity to produce a viable nuclear weapon.

Nuclear terrorism could include:

Nuclear terrorism, according to a 2011 report published by the Belfer Center for Science and International Affairs at Harvard University, can be executed and distinguished via four pathways:

Former U.S. President Barack Obama called nuclear terrorism "the single most important national security threat that we face". In his first speech to the U.N. Security Council, President Obama stated that "Just one nuclear weapon exploded in a city—be it New York or Moscow, Tokyo or Beijing, London or Paris—could kill hundreds of thousands of people", and warned such an attack could "destabilize our security, our economies, and our very way of life".

As early as December 1945, politicians worried about the possibility of smuggling nuclear weapons into the United States, though this was still in the context of a battle between the superpowers of the Cold War. Congressmen quizzed the "father of the atomic bomb", J. Robert Oppenheimer, about the possibility of detecting a smuggled atomic bomb:

Sen. Millikin: We... have mine-detecting devices, which are rather effective... I was wondering if anything of that kind might be available to use as a defense against that particular type of use of atomic bombs.
Dr. Oppenheimer: If you hired me to walk through the cellars of Washington to see whether there were atomic bombs, I think my most important tool would be a screwdriver to open the crates and look. I think that just walking by, swinging a little gadget would not give me the information.