In legal philosophy, legal positivism is the theory that the existence of the law and its content depend on social facts, such as acts of legislation, judicial decisions, and customs, rather than on morality. This contrasts with theories such as natural law, which hold that law is necessarily connected to morality in such a way that any law that contradicts morality lacks legal validity.

Thomas Hobbes defined law as the command of the sovereign. This idea was elaborated in the 18th and 19th centuries by legal philosophers such as Jeremy Bentham and John Austin, who argued that a law is valid not because it is intrinsically moral or just, but because it comes from the sovereign, is generally obeyed by the people, and is backed up by sanctions. Hans Kelsen developed legal positivism further by separating law not only from morality, as the early positivists did, but also from empirical facts, introducing the concept of a norm as an "ought" statement as distinct from a factual "is" statement. In Kelsen's view, the validity of a legal norm derives from a higher norm, creating a hierarchy that ultimately rests on a "basic norm": this basic norm, not the sovereign, is the ultimate source of legal authority. In addition to Kelsen, other prominent legal positivists of the 20th century include H. L. A. Hart and Joseph Raz.

The term positivism in legal positivism is connected to the sense of the verb to posit rather than the sense of positive (as opposed to negative). In this sense, the term positivism is derived from Latin positus, the past participle of ponere, meaning "to place" or "to put".^{[citation needed]} Legal positivism holds that laws are rules established (that is, "posited") by human beings, and that this act of positing the law makes it authoritative and binding.^{[better source needed]}

According to the positivist view, the source of a law is its enactment or recognition by a legal authority that is socially accepted and capable of enforcing its rules. The merits of a law are a separate issue from its legal validity: a law may be morally wrong or undesirable, but if it has been enacted by a legal authority with the power to do so, it is still a valid law.

Leslie Green summarises the distinction between merit and source: "The fact that a policy would be just, wise, efficient, or prudent is never sufficient reason for thinking that it is actually the law, and the fact that it is unjust, unwise, inefficient or imprudent is never sufficient reason for doubting it. According to positivism, law is a matter of what has been posited (ordered, decided, practiced, tolerated, etc.); as we might say in a more modern idiom, positivism is the view that law is a social construction."

Legal positivism does not claim that the laws so identified should be obeyed, or that there is necessarily value in having clear, identifiable rules (although some positivists may also make these claims). Indeed, the laws of a legal system may be quite unjust, and the government may be illegitimate; as a result, there may be no obligation to obey the law. Moreover, the fact that a law has been found to be valid by a court does not mean that the court should apply it in a particular case. As John Gardner has said, legal positivism is "normatively inert". It is a theory of law, not a theory of legal practice, adjudication, or political obligation, and legal positivists generally maintain that intellectual clarity is best achieved by separating these issues for independent analysis.

Analytical jurisprudence often distinguishes between two types of legal positivism: inclusive and exclusive legal positivism. The former accepts, whereas the latter denies, that there may be cases in which determining what the law is follows from considerations about what the law ought to be according to morality.

Both qualify as legal positivism because they share two basic tenets. First, the "social thesis": law is essentially a human creation and therefore its content is ultimately determined by social facts, such as acts of legislation, judicial decisions, and customs. Second, the "separation thesis": law and morality are conceptually distinct phenomena and therefore a norm can belong to the law even if is unjust or unfair.