Natural law (Latin: ius naturale, lex naturalis) is a system of law based on a close observation of human nature, and based on values intrinsic to human nature that can be deduced and applied independently of positive law (the express enacted laws of a state or society). According to the theory of law called jusnaturalism, all people have inherent rights, conferred not by act of legislation but by "God, nature, or reason." Natural law theory can also refer to "theories of ethics, theories of politics, theories of civil law, and theories of religious morality."

In Western tradition, it was anticipated by the pre-Socratics, for example in their search for principles that governed the cosmos and human beings. The concept of natural law was documented in ancient Greek philosophy, including Aristotle, and was referred to in ancient Roman philosophy by Cicero. References to it are also to be found in the Old and New Testaments of the Bible, and were later expounded upon in the Middle Ages by Christian philosophers such as Albert the Great and Thomas Aquinas. The School of Salamanca made notable contributions during the Renaissance.

Although the central ideas of natural law had been part of Christian thought since the Roman Empire, the foundation for natural law as a consistent system was laid by Aquinas, as he synthesised ideas from his predecessors and condensed them into his "Lex Naturalis" (lit. 'Natural law'). Aquinas argues that because human beings have reason, and because reason is a spark of the divine (see: image of God), all human lives are sacred and of infinite value compared to any other created object, meaning all humans are fundamentally equal and bestowed with an intrinsic basic set of rights that no human can remove.

Modern natural law theories took shape in the Age of Enlightenment, combining inspiration from Roman law, Christian scholastic philosophy, and contemporary concepts such as social contract theory. It was used in challenging the theory of the divine right of kings, and became an alternative justification for the establishment of a social contract, positive law, and government—and thus legal rights—in the form of classical republicanism. In the early decades of the 21st century, the concept of natural law is closely related to the concept of natural rights. Indeed, many philosophers, jurists and scholars use natural law synonymously with natural rights (Latin: ius naturale), or natural justice, though others distinguish between natural law and natural right.

Because of the intersection between natural law and natural rights, natural law has been claimed or attributed as a key component in the Act of Abjuration (1581) of the Netherlands, the Declaration of Independence (1776) of the United States, the Declaration of the Rights of Man and of the Citizen (1789) of France, the Universal Declaration of Human Rights (1948) of the United Nations, as well as the European Convention on Human Rights (1953) of the Council of Europe.