Karl Groos (10 December 1861, in Heidelberg – 27 March 1946, in Tübingen) was a German philosopher and psychologist who proposed an evolutionary instrumentalist theory of play. His 1898 book on The Play of Animals suggested that play is a preparation for later life.

Groos was full Professor of philosophy in Gießen, Basel and 1911–1929 in Tübingen.

His main idea was that play is basically useful, and so it can be explained by the normal process of evolution by natural selection. When animals 'play' they are practising basic instincts, such as fighting, for survival. This is translated from the original as "pre-tuning". Despite this insight, Groos' work is seldom read today, and his connection of play with aesthetics has been termed "misguided". Another area of study was the psychology of literature, including statistical analysis.

Among his scholars is the German philosopher Willy Moog (1888–1935) (doctorate on Goethe supervised by Karl Groos in Gießen 1909).