Tony Cliff (born Yigael Gluckstein; 20 May 1917 – 9 April 2000), was a Trotskyist activist. Born to a Jewish family in Palestine, he moved to Britain in 1947 and by the end of the 1950s had assumed the pen name of Tony Cliff. A founding member of the Socialist Review Group, which eventually became the Socialist Workers Party, in 1977 Cliff became effectively the leader.
Tony Cliff was born Yigael Gluckstein in Zikhron Ya'akov during World War I, the son of Esther and Akiva Gluckstein, Jewish immigrants from Poland. He had two brothers and a sister. He grew up in British-ruled Mandatory Palestine, and in his youth, he came to identify with Communism, though he never joined the Communist Party of Palestine, as he had not met any of its members before becoming a socialist activist. However, he did join the socialist-Zionist youth movement Hashomer Hatzair, and soon became not only a Trotskyist in 1933, but also a confirmed opponent of Zionism. Along with other Hashomer Hatzair members, he joined the illegal Palestine Revolutionary Communist League, necessitating the use of several pseudonyms in three languages.