Isaak Markovich Khalatnikov ForMemRS (Ukrainian: Ісаа́к Ма́ркович Хала́тников, Russian: Исаак Маркович Халатников; 17 October 1919 – 9 January 2021) was a leading Soviet theoretical physicist who made significant contributions to many areas of theoretical physics, including general relativity, quantum field theory, as well as the theory of quantum liquids.

Isaak Khalatnikov was born into a Ukrainian Jewish family in Yekaterinoslav (now Dnipro, Ukraine) and graduated from Dnipropetrovsk State University with a degree in Physics in 1941.

Much of Khalatnikov's research was a collaboration with, or inspired by, Lev Landau, including the Landau-Khalatnikov theory of superfluidity.

During 1969 he briefly worked as a part-time professor of theoretical physics at Leiden University.

[1] In 1970, inspired by the mixmaster model introduced by Charles W. Misner, then at Princeton University, Khalatnikov, together with Vladimir Belinski and Evgeny Lifshitz, introduced what has become known as the BKL conjecture, which is widely regarded as one of the most outstanding open problems in the classical theory of gravitation.