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 team at the University of Columbia, at Enrico Fermi's 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.