The monarchy of the United Kingdom (commonly referred to as the British monarchy) is the constitutional monarchy of the United Kingdom and its overseas territories. The present monarch, Elizabeth II, has reigned since 6 February 1952. She and her immediate family undertake various official, ceremonial and representational duties. As a constitutional monarch, the Queen is limited to non-partisan functions such as bestowing honours. Though the ultimate executive authority over the government of the United Kingdom is still by and through the monarch's royal prerogative, in practice these powers are only used according to laws enacted in Parliament or within the constraints of convention and precedent.

The British monarchy traces its origins from the kings of the Angles and the early Scottish kings. By the year 1000, the kingdoms of England and Scotland had resolved from the petty kingdoms of early medieval Britain. The last Anglo-Saxon monarch (Harold II) was defeated and killed in the Norman invasion of 1066 and the English monarchy passed to the Norman conquerors. In the thirteenth century, the principality of Wales was absorbed by England, and Magna Carta began the process of reducing the political powers of the monarch.

From 1603, when the Scottish king James VI inherited the English throne as James I, both kingdoms were ruled by a single monarch. From 1649 to 1660, the tradition of monarchy was broken by the republican Commonwealth of England that followed the War of the Three Kingdoms. 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 became nominal head of the vast British Empire, which covered a quarter of the world at its greatest extent in 1921.

In 1922, most of Ireland seceded from the Union as the Irish Free State, but in law the monarch remained sovereign there until 1949. In 1931, the unitary British monarchy throughout the empire was split into legally distinct crowns for each of the Commonwealth realms. After World War II, former colonies and dominions became independent of Britain, bringing the British Empire to an end. George VI and his successor, Elizabeth II, adopted the title Head of the Commonwealth as a symbol of the free association of the independent countries comprising the Commonwealth of Nations.

At present, 15 other independent Commonwealth countries share with the United Kingdom the same person as their monarch. As such, the terms British monarchy and British monarch are frequently still employed in reference not only to the extranational person and institution shared amongst all 16 of the realms, but also to the distinct monarchies within each of these countries, often at variance with the different, specific, and official national titles and terms for each jurisdiction.