Raj Kumar Pathria (born March 30, 1933) is a theoretical physicist and an Urdu poet.Pathria is known for his work on superfluidity in liquid helium, Lorentz transformation of thermodynamic quantities, a rigorous evaluation of lattice sums and finite-size effects in phase transitions.
Pathria is also the author of a graduate textbook, Statistical Mechanics, whose third edition appeared in 2011.
He also wrote a book on relativity that has been published in a Dover edition.