The Fifth Monarchists, or Fifth Monarchy Men, were a Protestant sect which advocated Millennialist views, active during the 1649 to 1660 Commonwealth of England. Named after a prophecy in the Book of Daniel that Four Monarchies would precede the Fifth or establishment of the Kingdom of God on earth, the group was one of a number of Nonconformist sects that emerged during the Wars of the Three Kingdoms. Perhaps its best known adherent was Major-General Thomas Harrison, executed in October 1660 as a regicide, while Oliver Cromwell was a sympathiser until 1653.

The Commonwealth was the political structure during the period from 1649 to 1660 when England and Wales, later along with Ireland and Scotland, were governed as a republic after the end of the Second English Civil War and the trial and execution of Charles I. The republic's existence was declared through "An Act declaring England to be a Commonwealth", adopted by the Rump Parliament on 19 May 1649. Power in the early Commonwealth was vested primarily in the Parliament and a Council of State. During the period, fighting continued, particularly in Ireland and Scotland, between the parliamentary forces and those opposed to them, in the Cromwellian conquest of Ireland and the Anglo-Scottish war of 1650–1652.