Justiciar is the English form of the medieval Latin term justiciarius or justitiarius ("man of justice", i.e. judge). During the Middle Ages in England, the Chief Justiciar (later known simply as the Justiciar) was roughly equivalent to a modern Prime Minister of the United Kingdom, as the monarch's chief minister. Similar positions existed in continental Europe, particularly in Norman Italy and in the Carolingian Empire.

A similar office was formed in Scotland, although there were usually two or three – the Justiciar of Scotia, the Justiciar of Lothian and, in the 13th century, the Justiciar of Galloway. These offices later evolved into a national one called Lord Justice-General. The modern title is Lord President of the Court of Session.

The Justiciar of Ireland was an office established during the Anglo-Norman invasion of Ireland and was a key tool in its colonisation. Following the conquest of the Principality of Wales in the 13th century, the areas that became personal fiefs of the English monarchs were placed under the control of the Justiciar of North Wales and the Justiciar of South Wales.