Abraham Klein (January 10, 1927 – June 20, 2003) was an American theoretical physicist. Klein studied at Brooklyn College (bachelor's degree in 1947) and at Harvard University, where he earned his master's degree in 1948 and doctorate in 1950 under Julian Schwinger. In 1955 he became associate professor at the University of Pennsylvania, where he received a full professorship in 1958 and retired in 1994.
Klein studied models of collective behavior in finite many-body systems, especially in nuclear physics, for example in Boson model and in an extension of the Hartree–Fock method with Robert Kerman (Kerman–Klein method). In the 1980s he occupied himself with, among other things, the interacting boson model, and in the 1970s with quantum field theory in strong fields (with Johann Rafelski).
He was a Sloan Fellow and Guggenheim Fellow, Honorary Doctor of Goethe University of Frankfurt, and Alexander von Humboldt Senior Scientist. Klein was a Fellow of the American Physical Society.
He was married and had two daughters.