Neptunium, whose name derives from the planet Neptune, was the first synthetic transuranian element to be discovered. Its isotope, Np 239, was produced by McMillan and Abelson in 1940, in Berkeley (University of California), as by-product of the bombardment of uranium with cyclotron neutrons. These investigators suspected that Np 239 should decay to another element with higher atomic number. This fact was confirmed with the discovery of the plutonium by Seaborg, McMillan, Kennedy and Wahl, in 1940.

The Np 237 isotope is obtained, in the order of kilograms, as by-product in nuclear reactors. L. B. Magnusson and T. J. LaChapelle obtained this isotope in measurable amounts for the first time in October of 1944, in the Metallurgy Laboratory of the University of Chicago, as product of beta decay of U 237.