The flag of the United Kingdom of Great Britain and Northern Ireland, commonly known as the Union Jack or Union Flag, is the national flag of the United Kingdom.

The current design of the Union Jack dates from the union of Ireland and Great Britain in 1801. It consists of the red cross of Saint George (patron saint of England), edged in white, superimposed on the Cross of St Patrick (patron saint of Ireland), which are superimposed on the Saltire of Saint Andrew (patron saint of Scotland). Wales, however, is not represented in the Union Flag by Wales's patron saint, Saint David, as at the time the flag was designed Wales was part of the Kingdom of England.

The flag's correct height-to-length proportions are 1:2. However, the version used by the British Army modifies the proportions to 3:5 and also crops two of the red diagonals.

The earlier flag of Great Britain was established in 1606 by a proclamation of King James VI and I of Scotland and England. The new flag of the United Kingdom was officially created by an Order in Council of 1801, reading as follows:

The Union Jack, or Union Flag, is the national flag of the United Kingdom. The flag also has an official or semi-official status in some other Commonwealth realms; for example, it is, by law, an official flag in Canada and known there as the Royal Union Flag. Further, it is used as an official flag in some of the smaller British overseas territories. The Union Jack also appears in the canton (upper left-hand quarter) of the flags of several nations and territories that are former British possessions or dominions.

The claim that the term Union Jack properly refers only to naval usage has been disputed, following historical investigations by the Flag Institute in 2013.

The origins of the earlier flag of Great Britain date back to 1606. James VI of Scotland had inherited the English and Irish thrones in 1603 as James I, thereby uniting the crowns of England, Scotland, and Ireland in a personal union, although the three kingdoms remained separate states. On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England (a red cross on a white background, known as St George's Cross), and the flag of Scotland (a white saltire on a blue background, known as the Saltire or St Andrew's Cross), would be joined together, forming the flag of England and Scotland for maritime purposes. King James also began to refer to a "Kingdom of Great Britaine", although the union remained a personal one.