Calgacus was the leader of the Caledonian Confederacy who fought the Roman army of Gnaeus Julius Agricola at the Battle of Mons Graupius in northern Scotland in AD 83 or 84. His name can be as interpreted as Celtic *calg-ac-os, "possessing a blade" or "possessing a penis".

The only historical source that features him is Tacitus' Agricola which describes him as 'the most distinguished for birth and valour among the chieftans'. Tacitus wrote a speech for him in advance of the battle in which he describes the exploitation of Britain by Rome and rouses his troops to fight. It is this speech which includes Tacitus's famous phrase, "where they make a desert, they call it peace."

Calgacus is not mentioned during or after the battle and he is not named as one of the hostages Agricola took with him after putting the Caledonians to flight. It is therefore unclear as to whether he was killed in combat or escaped to play a part in the uneasy peace of succeeding decades.