Grigory Aleksandrovich Margulis (, first name often given as Gregory, Grigori or Gregori; born February 24, 1946) is a Russian-American mathematician. He was born in Moscow. Margulis was known for his work on lattices in Lie groups, and the creation of methods from ergodic theory into diophantine approximation.
He was awarded a Fields Medal in 1978, a Wolf Prize in Mathematics in 2005, and an Abel Prize in 2020, becoming the fifth mathematician to receive the three prizes.
In 1991, he joined the faculty of Yale University. He is the Erastus L. De Forest Professor of Mathematics.