Millenarianism or millenarism is the belief that the end of the world is soon. This pattern of behaviour occurs in both religious and political systems.

Typically, a leader or teacher claims that the current society is corrupt or unjust and will be destroyed soon, and that followers must take action either to prevent it or to survive it. Often, the disaster or battle to come will be followed by a new, purified world in which believers will be rewarded.

Millenarian beliefs can make people ignore conventional rules of behaviour, which can result in violence directed inwards (such as mass suicides) and/or outwards (such as terrorist acts). It sometimes includes a belief in supernatural powers or predetermined victory.

Millenarian ideologies or religious sects often appear in oppressed peoples.

The words millenism, millennialism, millennianism, millenniarism and millenniumism are rarer synonyms.