Alfred Nier (1911 - 1994) was an American physicist.

Nier was a pioneer in the field of mass spectrometry. Using a mass spectrometer of his own design, Nier produced a sample of uranium-235 to be shipped to John Dunning's [1] team at the University of Columbia, at Enrico Fermi's [2] request. In doing so, Nier's team also showed that the uranium isotope responsible for nuclear fission was uranium-235 rather than uranium-238. He later worked at the Kellex Corporation to develop mass spectrographs that were used for the production of the American atomic bombs.