Demise of the Crown is the legal term in the United Kingdom and the other Commonwealth realms for the transfer of the Crown upon the death or abdication of the monarch. The Crown transfers automatically to the monarch's heir. The concept evolved in the kingdom of England, and was continued in Great Britain and then the United Kingdom. The concept also became part of the constitutions of the British colonies, and was continued in the constitutions of the Commonwealth realms, until modified within those realms.

Originally, the demise of the Crown in England had significant legal effects: individuals who had been appointed to office by the deceased monarch lost their positions; if Parliament was sitting, it automatically dissolved; and actions in the royal courts automatically discontinued and had to be re-started. Almost all of these legal effects have been abolished by statutes of the British Parliament and the parliaments of the Commonwealth realms, so that the demise of the Crown no longer has much legal significance.

Although the concept of the demise of the Crown was originally based on the monarch's death, it was used in 1936 to describe the transfer of the Crown to George VI upon the abdication of Edward VIII, as the Act of Parliament that gave legal effect to the abdication specified that, upon the instrument of abdication taking effect, Edward VIII would cease to be King and there would be a demise of the Crown.

Other monarchies use different terminologies for the end of a reign.

If the monarch who dies or abdicates is the Queen and her successor is the King, references to 'Queen's' automatically changes to 'King's'. For example, lawyers in the Dominions of the Crown who are Queen's Counsel (Q.C.) at the time of the demise become King's Counsel (K.C.) without any need to seek new letters patent. The reverse is also true, as was the case from 6 February 1952 upon the death of King George VI, when living King's Counsel became Queen's Counsel.

The term "demise" was originally a law French term used for the transfer of real property, whether by lease or sale. Blackstone notes that this term for the transfer of property then became used on the death of the monarch to mean "the kingdom is transferred or demised to his successor". "Demise" in this sense is not referring to the monarch's death, but to the automatic transfer of the monarchy to the heir, by operation of law:

A THIRD attribute of the king's majesty is his perpetuity. The law ascribes to him, in his political capacity, an absolute immortality. The king never dies. Henry, Edward, or George may die; but the king survives them all. For immediately upon the decease of the reigning prince in his natural capacity, his kingship or imperial dignity, by act of law, without any interregnum or interval, is vested at once in his heir, who is, eo instanti, king to all intents and purposes.

The historical effects of the demise of the Crown were the result of the personal nature of the monarch as head of government in the medieval period. The legislative, executive and judicial functions of government all flowed from the monarch's personal authority. The monarch's death therefore affected all aspects of the monarchical government.