An heir apparent, sometimes femininely heiress apparent, frequently regularly heir, is a person who is first in an order of succession and cannot be displaced from inheriting by the birth of another person; a person who is first in the order of succession but can be displaced by the birth of a more eligible heir is known as heir presumptive.

Today these terms most commonly describe heirs to hereditary titles (e.g. titles of nobility) or offices, especially when only inheritable by a single person. Most monarchies refer to the heir apparent of their thrones with the descriptive term of crown prince or crown princess, but they may also be accorded with a more specific substantive title: such as Prince of Orange in the Netherlands, Duke of Brabant in Belgium, Prince of Asturias in Spain (also granted to heirs presumptive), or the Prince of Wales in the United Kingdom; former titles include Dauphin in the Kingdom of France, and Tsesarevich in Imperial Russia.

The term is also used metaphorically to indicate an expected successor to any position of power, e.g. a political or corporate leader.

This article primarily describes the term heir apparent in a hereditary system regulated by laws of primogeniture—it may be less applicable to cases where a monarch has a say in naming the heir (performed either while alive, e.g. crowning the heir as a rex iunior, or through the monarch's will).