Millennialism (from millennium, Latin for "a thousand years") or chiliasm (from the Greek equivalent) is a belief advanced by some religious denominations that a Golden Age or Paradise will occur on Earth prior to the final judgment and future eternal state of the "World to Come".

Christianity and Judaism have both produced messianic movements which featured millennialist teachings—such as the notion that an earthly kingdom of God was at hand. These millenarian movements often led to considerable social unrest.

Similarities to millennialism appear in Zoroastrianism, which identified successive thousand-year periods, each of which will end in a cataclysm of heresy and destruction, until the final destruction of evil and of the spirit of evil by a triumphant king of peace at the end of the final millennial age. "Then Saoshyant makes the creatures again pure, and the resurrection and future existence occur" (Zand-i Vohuman Yasht 3:62).

Scholars have also linked various other social and political movements, both religious and secular, to millennialist metaphors.