Nicolai Hartmann (February 20, 1882 – October 9, 1950) was a German philosopher.

Hartmann was born, of German descent, in Riga, now Latvia and then capital of the Russian province of Livonia. He studied Medicine at the University of Tartu (then Jurjev), then Philosophy in St. Petersburg and, most importantly, at the University of Marburg in Germany, where he took his Ph.D. and Habilitation. He was professor of philosophy in Marburg (1922–25), Cologne (1925–31), Berlin (1931–45) and Göttingen (1945–50), where he died.

Originally a Neokantian, studying under Hermann Cohen and Paul Natorp Hartmann soon developed his own philosophy which has been variously described as a variety of existentialism, critical realism, etc. Hartmann suffered from the comparison with, and popularity of, his Marburg successor Martin Heidegger, who was regarded as being much more radical and "cool". Among Hartmann's students are Boris Pasternak and Hans-Georg Gadamer. His brilliant and encyclopedic work is basically forgotten today; although world famous during his heydays, he is a typical insider's philosopher. An exception may be his early work on the philosophy of biology, which is currently regaining popularity due to its high relevance for the discussion about genomics, cloning, and the like.

Works (in English):

• Ethics, 3 vols., 1932
• New Ways of Ontology, 1952