A national myth is an inspiring narrative or anecdote about a nation's past. Such myths often serve as important national symbols and affirm a set of national values. A myth is entirely fictitious but it is often mixed with aspects of historic reality to form a mythos, which itself has been described as "a pattern of beliefs expressing often symbolically the characteristic or prevalent attitudes in a group or culture". Myths, or mythoi, thereby operate in a specific social and historical setting that help structure national imagination and identity. A national myth may take the form of a national epic, or it may be incorporated into a civil religion. Mythos derives from μῦθος, Greek for "myth".

A national myth is a narrative which has been elevated to a serious symbolic and esteemed level so as to be true to the nation.^{[verification needed]} The national folklore of many nations includes a founding myth, which may involve a struggle against colonialism or a war of independence or unification. In many cases, the meaning of the national myth is disputed among different parts of the population. In some places, the national myth may be spiritual and refer to stories of the nation's founding by one or several gods, leaders favored by gods, or other supernatural beings.

National myths often exist only for the purpose of state-sponsored propaganda. In totalitarian dictatorships, the leader might be given, for example, a mythical supernatural life history in order to make them seem god-like and supra-powerful (see also cult of personality). In liberal regimes they can inspire civic virtue and self-sacrifice or consolidate the power of dominant groups and legitimate their rule.

The concept of national identity is inescapably connected with myths. A complex of myths is at the core of nationalistic ethnic identity. Some scholars believe that national identities, supported by invented histories, were constructed only after national movements and national ideologies emerged.

All modern national identities were preceded by nationalist movements.^{[verification needed]} Although the term "nation" was used in the Middle Ages, it had usually an ethnic meaning and seldom referred to a state. In the age of nationalism, it was linked to efforts aimed at creating nation-states.

National myths foster national identities. They are important tools of nation-building, which can be done by emphasizing differences between people of different nations. They can cause conflict as they exaggerate threats posed by other nations and minimize the costs of war.

The nationalist myth of a stable homeland community is explained psychoanalytically as the result of the complexity of relations within the modern external world and the incoherence of one's inner psychological world. Nationalist identity facilitates imagined stability.

National myths are created and propagated by national intellectuals, and they can be used as instruments of political mobilization on demographic bases such as ethnicity.