Arthur Koestler CBE (5 September 1905, Budapest – 3 March 1983, London) was a Jewish-Hungarian polymath author who became a naturalized British subject. In 1931, he joined the Communist Party, but left the party seven years later, after emigrating to the United Kingdom. By the late 1940s, he was one of the most recognized and outspoken British anti-communists. He wrote numerous books, of which the most famous is the novel Darkness at Noon about the Great Purge in the Soviet Union, an indictment of Stalinism.

He was born Kösztler Artúr (Hungarian names have the surname first) in Budapest, Austria-Hungary, to a German-speaking Hungarian family of Ashkenazi Jewish descent. His father, Henrik, was a prosperous start-up industrialist and inventor. His great business success was a "health" soap, which substituted conventional soaps based on animal fats (scarce during the WWI). Henrik's mineral soaps were thought to have health qualities thanks to their weak radioactivity, which in those times was considered curative. When Artur was 14, his family moved to Vienna. It was at this age which he had a "mystical experience", which perhaps gave rise to his later interest in the paranormal.

Koestler studied science and psychology at the University of Vienna, where he became President of a Zionist student fraternity. A month before he was due to finish his studies, he burnt his matriculation book and did not take his final examinations but made "aliyah" to Israel (then a British Mandate). From 1926 to 1929 he lived in the British Mandate of Palestine, firstly in a kibbutz in the Jezreel Valley ("Heftzibah"), and later in Tel Aviv and Jerusalem, where he almost starved. He left Palestine for Paris as a correspondent to the Ullstein-Verlag group of German newspapers. A year later he became science editor for Ullstein based in Berlin; a highlight of that post was membership in a 1931 Zeppelin expedition to the North Pole.