Ultranationalism or extreme nationalism is an extreme form of nationalism in which a country asserts or maintains detrimental hegemony, supremacy, or other forms of control over other nations (usually through violent coercion) to pursue its specific interests. Ultranationalist entities have been associated with the engagement of political violence even during peacetime. The belief system has also been cited as the inspiration behind acts of organized mass murder in the context of international conflict, e.g. the Cambodian genocide.

In ideological terms, scholars such as British political theorist Roger Griffin have found that ultranationalism arises from seeing modern nation-states as living organisms directly akin to physical people such that they can decay, grow, die, and additionally experience rebirth. Political campaigners have divided societies in stark mythological ways between those perceived as degenerately inferior and those perceived as a part of a great cultural destiny. Ultranationalism is an aspect of fascism, with historic governments such as the regimes of Nazi Germany, Francoist Spain and Fascist Italy building on ultranationalist foundations using specific plans of supposed widespread national renewal.