Isaac Deutscher (Polish: Izaak Deutscher; 3 April 1907 – 19 August 1967) was a Polish Marxist writer, journalist and political activist who moved to the United Kingdom before the outbreak of World War II. He is best known as a biographer of Leon Trotsky and Joseph Stalin and as a commentator on Soviet affairs.

His scholarly biographies of Trotsky and Stalin have received widespread acclaim, with several reviewers ranking his three-volume Prophet biography of Trotsky among the greatest of political biographies; some criticism has been directed at his overly sympathetic tone for the subject matter. His biography of Trotsky was also highly influential among the British New Left in the 1960s and 1970s.

Deutscher was born in Chrzanów, a town in the Galicia region of the Austro-Hungarian Empire (now in southern Poland), into a family of religiously observant Jews. He studied with a Hasidic rebbe and was acclaimed as a prodigy in the study of the Torah and the Talmud. He lived through three pogroms in 1918 that followed the collapse of the Austro-Hungarian empire. By the time of his bar mitzvah, however, he had lost his faith. He "tested God" by eating non-kosher food at the grave of a tzadik (holy person) on Yom Kippur. When nothing happened, he became an atheist. Deutscher first attracted notice as a poet, when he began publishing poems in Polish literary periodicals at the age of sixteen. His verse, in Yiddish and Polish, concerned Jewish and Polish mysticism, history and mythology, and he attempted to bridge the gulf between the Polish and Yiddish cultures. He also translated poetry from Hebrew, Latin, German, and Yiddish into Polish.

Deutscher studied literature, history, and philosophy as an extramural student at the Jagiellonian University in Kraków. Soon he left Kraków for Warsaw, where he studied philosophy, economics and Marxism. Around 1927, he joined the illegal Communist Party of Poland (KPP) and became the editor of the party's underground press. He wrote for the Jewish Nasz Przegląd ("Our Review") and for the Marxist Miesięcznik Literacki ("The Literary Monthly"). In 1931 he toured the Soviet Union, seeing the economic conditions under the first Five Year Plan. The University of Moscow and the University of Minsk offered him posts as professor of history of socialism and of Marxist theory, but he declined the offers and returned to Poland. Deutscher co-founded the first anti-Stalinist group in the Communist Party of Poland, protesting the party view that Nazism and social democracy were "not antipodes but twins." This contradicted the then official communist line, according to which social democrats were "social fascists", the greatest enemies of the communist party. In an article "The Danger of Barbarism over Europe", Deutscher urged the formation of a united front of socialists and communists against Nazism. He was expelled from the KPP in 1932, officially for "exaggerating the danger of Nazism and spreading panic in the communist ranks."

In April 1939, Deutscher left Poland for London as a correspondent for a Polish-Jewish newspaper for which he had worked as a proof reader for fourteen years. This move saved his life and paved the way for his future career. He never returned to Poland and never saw any of his family again.

Deutscher had rejected the formation by Trotsky in 1938 of the Fourth International. In England he briefly wrote for, but never joined, the Trotskyist Revolutionary Workers League. He wrote that he "left Warsaw with the crystallised view that the Fourth International was a complete fiasco, that it was degenerating rapidly into an arid sect with which I did not want to be involved." Of the RWL, he wrote: "Although all of its members were sincere and good people, their political thinking was so infantile that it only confirmed me in my view that the Fourth International was a non-starter... Apart from Trotsky himself the Fourth International was a zero."

Germany invaded Poland in September 1939 and Deutscher's connection with his newspaper was severed. He taught himself English and began writing for English magazines. He was soon a regular correspondent for the leading weekly The Economist.

In 1940, he joined the Polish Army in Scotland, but was interned as a dangerous subversive. Released in 1942, he joined the staff of The Economist and became its expert on Soviet affairs and military issues, and its chief European correspondent. He also wrote for The Observer as a roving European correspondent under the pen-name "Peregrine". He was one of the so-called Shanghai Club (named after a restaurant in Soho) of left-leaning and emigre journalists that included Sebastian Haffner (also on The Observer), E. H. Carr, George Orwell, Barbara Ward and Jon Kimche.