Leonid Veniaminovich Keldysh (Russian: Леонид Вениаминович Келдыш; 7 April 1931 – 11 November 2016) was a Soviet and Russian physicist. Keldysh was a professor in the I.E. Tamm Theory division of the Lebedev Physical Institute of the Russian Academy of Sciences in Moscow and a faculty member at Texas A&M University. He is known for developing the Keldysh formalism,^[2] a powerful quantum field theory framework designed to describe a system in a non-equilibrium state, as well as for the theory of excitonic insulators (Keldysh-Kopaev model, with Yuri Kopaev).^[3] Keldysh's awards include the 2009 Rusnanoprize, an international nanotechnology award, for his work related to molecular-beam epitaxy,^[4] the 2011 Evgenii Feinberg Memorial Medal, and the 2015 Lomonosov Grand Gold Medal of the Russian Academy of Sciences.^[5]

Keldysh was a son of mathematician Lyudmila Keldysh. His uncle, Mstislav Keldysh, was a mathematician and the president of the Academy of Sciences of the Soviet Union. Sergei Novikov, a mathematician and a Fields medalist, is his step-brother.^[6]