Cromwellian is an adjective relating to Oliver Cromwell (1599–1658), Lord Protector of the Commonwealth of England, and his era

Cromwellian may also refer to:

• Roundhead, a member of the Parliamentarian side of the English Civil War against the Royalists
• Cromwellians, supporters of the Cromwellian Protectorate during the English Interregnum
• The Cromwellian, a 1960s nightclub in London, England, UK

• Cromwell (disambiguation)
• Oliver Cromwell (disambiguation)