Kazimierz Radosław Elehard baron Kelles-Krauz (22 March 1872 – 24 June 1905) was a Polish philosopher and sociologist, member of the Polish Socialist Party. He was one of the most significant Marxist thinkers at the end of the 19th century.

Kelles-Krauz was born in Szczebrzeszyn, Russian Empire and died in Pernitz, Austria-Hungary.

His greatest contribution to sociology is the "law of retrospective revolution" according to which "the ideals with which each reform movement tries to replace existing social norms are always similar to the norms of a more or less distant past".

Yale's Timothy Snyder argues that Kelles-Krauz, writing two decades before Hans Kohn and Carlton Hayes, ought to be among the small cluster of turn-of-the-century thinkers regarded as the pioneers of the modern study of nationalism.

He was the son of nobleman and landowner Michał Wilhelm Elehard Kelles-Krauz [pl] and Matylda Daniewska. He had three younger brothers, Jan Jakub, Bohdan, Stanisław Maciej and sister Matylda.^[1] His youngest brother Stanisław Maciej [pl] was also a PPS activist, senator of the Second Polish Republic (1928-30) and Polish ambassador to Denmark after the Second World War, married to PPS activist Maria Helena Nynkowska [pl].^[1]

Kazimierz married PPS activist Maria Katarzyna Goldsteyn [pl], with whom he had a daughter, Janina, a workers' activist and employee of the Ossolineum.^[1]