Masatoshi Nei (根井正利, Nei Masatoshi) (born January 2, 1931) is a Japanese-born American evolutionary biologist currently affiliated with the Department of Biology at Temple University as a Carnell Professor. He was, until recently, Evan Pugh Professor of Biology at Pennsylvania State University and Director of the Institute of Molecular Evolutionary Genetics; he was there from 1990 to 2015. Together with Walter Fitch, Nei co-founded the journal Molecular Biology and Evolution in 1983 and the Society for Molecular Biology and Evolution in 1993.