The Open Society and Its Enemies is a work on political philosophy by the philosopher Karl Popper, in which the author presents a defence of the open society against its enemies, and offers a critique of theories of teleological historicism, according to which history unfolds inexorably according to universal laws. Popper indicts Plato, Hegel, and Marx for relying on historicism to underpin their political philosophies.

Written during World War II, The Open Society and Its Enemies was published in 1945 in London by Routledge in two volumes: "The Spell of Plato" and "The High Tide of Prophecy: Hegel, Marx, and the Aftermath". A one-volume edition with a new introduction by Alan Ryan and an essay by E. H. Gombrich was published by Princeton University Press in 2013. The work was listed as one of the Modern Library Board's 100 Best Nonfiction books of the 20th century.

The book critiques historicism and defends the open society and liberal democracy. Popper argues that Plato's political philosophy has dangerous tendencies towards totalitarianism, contrary to the benign idyll portrayed by most interpreters. He praises Plato's analysis of social change but rejects his solutions, which he sees as driven by fear of change brought about by the rise of democracies, and as contrary to the humanitarian and democratic views of Socrates and other thinkers of the Athenian "Great Generation". Popper also criticizes Hegel, tracing his ideas to Aristotle and arguing that they were at the root of philosophical underpinnings of 20th century totalitarianism. He agrees with Schopenhauer's view that Hegel "was a flat-headed, insipid, nauseating, illiterate charlatan, who reached the pinnacle of audacity in scribbling together and dishing up the craziest mystifying nonsense." Popper criticizes Marx at length for his historicism, which he believes led him to overstate his case, and rejects his radical and revolutionary outlook. Popper advocates for direct liberal democracy as the only form of government that allows institutional improvements without violence and bloodshed.

Popper's book is divided into two volumes, which were only published as one volume in more recent editions. The first volume, The Spell of Plato, focuses on Plato, and the second volume, The High Tide of Prophecy, focuses on Karl Marx.

The first section, "The Myth of Origin and Destiny", contains chapters 1 through 3. The second section, "Plato's Descriptive Sociology", contains chapters 4 and 5. The third section, "Plato's Political Programme", contains chapters 6 through 9. The fourth and last section of the first volume, "The Background of Plato's Attack", only contains chapter 10. The volume is closed by an unnumbered section of "Addenda".

Chapter 1, "Historicism and the Myth of Destiny", describes the historicist approach to social sciences, which Popper considers to be a flawed method. Historicism is "the doctrine that history is controlled by specific historical or evolutionary laws whose discovery would enable us to prophesy the destiny of man." Popper argues that the historicist approach gives poor results, and he outlines a method that he believes would yield better results. He also discusses how historicism originated and became entrenched in our culture. Popper provides "the doctrine of the chosen people" as an example of historicism. The doctrine assumes that God has chosen one people to be the selected instrument of His will, and that this people will inherit the earth – the law of historical development is laid down by the will of God. Popper argues that "the doctrine of the chosen people" grew out of the tribal form of social life, and he notes that other forms of historicist theories may still retain an element of collectivism. Popper identifies the two most important modern versions of historicism as the historical philosophy of racialism or fascism on the right and the Marxian historical philosophy on the left. Both theories base their historical forecasts on an interpretation of history which leads to the discovery of a law of its development.

Chapter 2, "Heraclitus", focuses on the emergence of historicism in ancient Greece, specifically through the philosophy of Heraclitus. Heraclitus believed in the idea of universal change and hidden destiny enforced by punishment, and saw strife or war as the dynamic and creative principle of all change, especially in social life. His philosophy was inspired by his personal experiences of social and political disturbances, and his emphasis on the importance of social dynamics reflects this. Popper sees Heraclitus as a predecessor to Hegel.

Chapter 3, "Plato's Theory of Forms or Ideas", discusses the eponymous theory. It is longer than the first two chapters, and is divided by Popper into six parts. According to Popper, Plato's philosophy was heavily influenced by his experiences of political turmoil and instability, and he believed that moral degeneration leads to political degeneration. He strove to arrest this political change by establishing a state that does not degenerate or change, which he saw as the perfect state of the Golden Age. While Plato deviated from the tenets of historicism by believing in an ideal state that does not change, he shared with Heraclitus a tendency to shrink back from the last consequences of historicism.