Lev Isaakovich Shestov (Russian: Лев Исаакович Шестов; 5 February [O.S. 24 January] 1866 – 19 November 1938), born Yeguda Lev Shvartsman (Russian: Иегуда Лейб Шварцман), was a Russian existentialist and religious philosopher. He is best known for his critiques of both philosophical rationalism and positivism. His work advocated a movement beyond reason and metaphysics, arguing that these are incapable of conclusively establishing truth about ultimate problems, including the nature of God or existence. Contemporary scholars have associated his work with the label "anti-philosophy."

Shestov wrote extensively on philosophers such as Nietzsche and Kierkegaard, as well as Russian writers such as Dostoyevsky, Tolstoy, and Chekhov. His published books include Apotheosis of Groundlessness (1905) and his magnum opus Athens and Jerusalem (1930–37). After emigrating to France in 1921, he befriended and influenced thinkers such as Edmund Husserl, Benjamin Fondane, Rachel Bespaloff, and Georges Bataille. He lived in Paris until his death in 1938.

Shestov was born Yeguda Lev Shvartsman in Kiev into a Jewish family. He was a cousin of Nicholas Pritzker, a lawyer who emigrated to Chicago and became the patriarch of the Pritzker family that is prominent in business and politics. He obtained an education at various places, due to fractious clashes with authority. He went on to study law and mathematics at the Moscow State University but after a clash with the Inspector of Students he was told to return to Kiev, where he completed his studies.

Shestov's dissertation was rejected by the St. Vladimir's Imperial University of Kiev on account of the revolutionary tendencies it expressed, thus preventing him from becoming a doctor of law. In 1898 he entered a circle of prominent Russian intellectuals and artists which included Nikolai Berdyaev, Sergei Diaghilev, Dmitri Merezhkovsky and Vasily Rozanov. Shestov contributed articles to a journal the circle had established. During this time he completed his first major philosophical work, Good in the Teaching of Tolstoy and Nietzsche: Philosophy and Preaching; two authors profoundly impacting Shestov's thought.

He developed his thinking in a second book on Fyodor Dostoyevsky and Friedrich Nietzsche, which increased Shestov's reputation as an original and incisive thinker. In All Things Are Possible (published in 1905) Shestov adopted the aphoristic style of Friedrich Nietzsche to investigate the difference between Russian and European Literature. Although on the surface it is an exploration of numerous intellectual topics, at its base it is a sardonic work of Existentialist philosophy which both criticizes and satirizes our fundamental attitudes towards life situations. D. H. Lawrence, who wrote the foreword to S.S. Koteliansky's literary translation of the work, summarized Shestov's philosophy with the words: " 'Everything is possible' - this is his really central cry. It is not nihilism. It is only a shaking free of the human psyche from old bonds. The positive central idea is that the human psyche, or soul, really believes in itself, and in nothing else". Shestov deals with key issues such as religion, rationalism, and science in this highly approachable work, topics he would also examine in later writings such as In Job's Balances. Shestov's own key quote from this work is probably the following: "...we need to think that only one assertion has or can have any objective reality: that nothing on earth is impossible. Every time someone wants to force us to admit that there are other, more limited and limiting truths, we must resist with every means we can lay hands on".

Shestov's works were not met with approval even by some of his closest Russian friends. Many saw in Shestov's work a renunciation of reason and metaphysics, and even an espousal of nihilism. Nevertheless, he would find admirers in such writers as D. H. Lawrence and his friend Georges Bataille.

In 1908 Shestov moved to Freiburg, Germany, and he stayed there until 1910, when he moved to a small Swiss village named Coppet. During this time the author worked prolifically. One of the fruits of these labours was the publication of Great Vigils and Penultimate Words. He returned to Moscow in 1915, and in this year his son Sergei died in combat against the Germans. During the Moscow period, his work became more influenced by matters of religion and theology. The seizure of government by the Bolsheviks in 1917 made life difficult for Shestov, and the Marxists pressured him to write a defence of Marxist doctrine as an introduction to his new work, Potestas Clavium; otherwise it would not be published. Shestov refused this, yet with the permission of the authorities he lectured at the University of Kiev on Greek philosophy.

Shestov's dislike of the Soviet regime led him to undertake a long journey out of Russia, and he eventually ended up in France. The author was a popular figure in France, where his originality was quickly recognized. In Paris, he soon befriended, and much influenced, the young Georges Bataille. He was also close to Eugene and Olga Petit, who helped him and his family relocate to Paris and integrate into the French political and literary circles. That this Russian was newly appreciated is attested by his having been asked to contribute to a prestigious French philosophy journal. In the interwar years, Shestov continued to develop into a thinker of great prominence. During this time he had become totally immersed in the study of such great theologians as Blaise Pascal and Plotinus, whilst at the same time lecturing at the Sorbonne in 1925. In 1926 he was introduced to Edmund Husserl, with whom he maintained a cordial relationship despite radical differences in their philosophical outlook. In 1929, during a return to Freiburg he met with Edmund Husserl, and was urged to study Danish philosopher Søren Kierkegaard.