Neptunium (Np) is a chemical element with atomic number 92. It is named after the planet Neptune, and is only found in negligible trace amounts in nature.

Neptunium is an intermediate step in converting uranium into plutonium. Uranium is first turned into neptunium, which then decays into plutonium. The isotope neptunium-237, similarly to uranium-235, is fissile and could theoretically be used in nuclear weapons.