Gideon Klein (6 December 1919 – c. January 1945) was a Czech pianist, classical music composer, and organizer of cultural life at Theresienstadt concentration camp.
Klein was born into a Moravian Jewish family in Přerov and, showing musical talent early, studied piano with Růžena Kurzová and Vilém Kurz, and composition with Alois Hába (in 1939–40). He was forced to abort his university studies in 1940 when the Nazis closed all institutions of higher learning following their occupation of Czechoslovakia in March 1939. Since compositions and performances by Jewish musicians were banned, his own works could not be performed, though he managed to perform as a concert pianist under several aliases for a time, e.g., under the pseudonym Karel Vranek. In 1940 he was offered a scholarship at the Royal Academy of Music in London, but by that time anti-Jewish legislation prevented his emigration.
In December 1941 he was deported by the Nazis to Terezín concentration camp, where along with Leoš Janáček's pupil Pavel Haas, Hans Krása, and Schoenberg's pupil Viktor Ullmann he became one of the major composers at that camp. he initially gave concerts in secret, but the camp became one of the few in which artistic activity was eventually permitted by Nazis on any scale, if only to deceive the broader public as to their real intentions. His works from these years include music for string quartet (similar in tone to Berg's opus 3 work), a string trio, and a piano sonata, among others. Moreover, Klein performed as solo pianist at approximately 15 recitals, and also participated in chamber music performances (member of piano trio, piano quartet).