A peremptory norm (also called jus cogens, Latin for "compelling law") is a fundamental principle of international law considered to have acceptance among the international community of states as a whole. Unlike ordinary customary law that has traditionally required consent and allows the alteration of its obligations between states through treaties, peremptory norms cannot be violated by any state. Under the Vienna Convention on the Law of Treaties, any treaty in violation of a peremptory norm is null and void. The treaty allows for the emergence of new peremptory norms, but does not itself specify any peremptory norms.

The number of peremptory norms is considered limited but not exclusively catalogued. They are not listed or defined by any authoritative body, but arise out of case law and changing social and political attitudes. Generally included are prohibitions on waging aggressive war, piracy, genocide, slavery, and torture.

Despite the seemingly clear weight of condemnation of such practices, some critics disagree with the division of international legal norms into a hierarchy. There is also disagreement over how such norms are recognized or established. The relatively new concept of peremptory norms is at odds with the traditionally consensual nature of international law considered necessary to state sovereignty. However, sovereignty has never truly been an absolute concept and limitations upon it are increasingly supported.

Peremptory norms also differ from traditional international law in that they are considered to be enforceable against not only states, but individuals as well. This has been increasingly accepted since the Nuremberg Trials (the first enforcement in world history of international norms upon individuals) and now might be considered uncontroversial.

There are often disagreements over whether a particular case violates a peremptory norm, and as in most international law, there is no court or body which can authoritatively answer such questions without the consent of the parties. As in other areas of law, states generally reserve the right to interpret the concept for themselves.