Masatoshi Nei (根井正利 Nei Masatoshi^?) is Evan Pugh Professor of Biology at Pennsylvania State University and Director of the Institute of Molecular Evolutionary Genetics since 1990.

He was born in 1931 in Miyazaki Prefecture, on Kyūshū Island, Japan. He was associate professor and professor of biology at Brown University from 1969 to 1972 and professor of population genetics at the Center for Demographic and Population Genetics, University of Texas at Houston, from 1972 to 1990.

He is a theoretical population geneticist and evolutionary biologist. Acting alone or working with his students, he has continuously developed new statistical theories of molecular evolution taking into account frontier knowledge of molecular biology. He has also developed several new concepts of evolutionary theory and proposed the theory of mutation-driven evolution.