Plutonium (Pu) is a radioactive chemical element with atomic number 94. It is named after the Pluto, following the pattern of planet based naming set out by uranium and neptunium. Plutonium is only found in extremely small amounts in nature and is therefore instead synthesized from uranium.

Both plutonium and uranium can be used in nuclear weapons. The plutonium-239 isotope is even more fissile than uranium-235, meaning less of it is needed to create a nuclear bomb. Unlike the Hiroshima bomb, which as uranium-based, the Nagasaki bomb was made of plutonium.