The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the constitutional form of government by which a hereditary sovereign reigns as the head of state of the United Kingdom, the Crown Dependencies (the Bailiwick of Guernsey, the Bailiwick of Jersey and the Isle of Man) and the British Overseas Territories. The current monarch is King Charles III, who ascended the throne on 8 September 2022, upon the death of his mother, Queen Elizabeth II.

The monarch and their immediate family undertake various official, ceremonial, diplomatic and representational duties. As the monarchy is constitutional, the monarch is limited to functions such as bestowing honours and appointing the prime minister, which are performed in a non-partisan manner. The sovereign is also able to comment on draft laws which directly affect the monarchy. The monarch is also Head of the British Armed Forces. Though the ultimate executive authority over the government is still formally by and through the royal prerogative, these powers may only be used according to laws enacted in Parliament and, in practice, within the constraints of convention and precedent. The Government of the United Kingdom is known as His Majesty's Government.

The British monarchy traces its origins from the petty kingdoms of Anglo-Saxon England and early medieval Scotland, which consolidated into the kingdoms of England and Scotland by the 10th century. England was conquered by the Normans in 1066, after which Wales also gradually came under the control of Anglo-Normans. The process was completed in the 13th century when the Principality of Wales became a client state of the English kingdom. Meanwhile, Magna Carta began the process of reducing the English monarch's political powers. In the 16th century, English and Scottish monarchs played a central role in what became the religious English Reformation and Scottish Reformation. From 1603, the English and Scottish kingdoms were ruled by a single sovereign. From 1649 to 1660, the tradition of monarchy was broken by the republican Commonwealth of England, which followed the Wars of the Three Kingdoms. Following the installation of William III and Mary II as co-monarchs in the Glorious Revolution, the Bill of Rights 1689, and its Scottish counterpart the Claim of Right Act 1689, further curtailed the power of the monarchy and excluded Roman Catholics from succession to the throne. In 1707, the kingdoms of England and Scotland were merged to create the Kingdom of Great Britain, and in 1801, the Kingdom of Ireland joined to create the United Kingdom of Great Britain and Ireland. The British monarch was the nominal head of the vast British Empire, which covered a quarter of the world's land area at its greatest extent in 1921.

The Balfour Declaration of 1926 recognised the evolution of the Dominions of the Empire into separate, self-governing countries within a Commonwealth of Nations. In the years after the Second World War, the vast majority of British colonies and territories became independent, effectively bringing the Empire to an end. George VI and his successors, Elizabeth II and Charles III, adopted the title Head of the Commonwealth as a symbol of the free association of its independent member states. The United Kingdom and fourteen other independent sovereign states that share the same person as their monarch are called Commonwealth realms. Although the monarch is shared, each country is sovereign and independent of the others, and the monarch has a different, specific, and official national title and style for each realm. Although the term is rarely used today, the fifteen Commonwealth realms are in personal union.