The Union Jack, or Union Flag, is the de facto national flag of the United Kingdom. Although no law has been passed making the Union Flag the official national flag of the United Kingdom, it has effectively become such through precedent. It is sometimes asserted that the term Union Jack properly refers only to naval usage, but this assertion was dismissed by the Flag Institute in 2013 following historical investigations. The flag has official status in Canada, by parliamentary resolution, where it is known as the Royal Union Flag. It is the national flag of all British overseas territories, being localities within the British state, or realm, although local flags have also been authorised for most, usually comprising the blue or red ensign with the Union Flag in the canton and defaced with the distinguishing arms of the territory. These may be flown in place of, or along with (but taking precedence after) the national flag. Governors of British Overseas Territories have their own personal flags, which are the Union Flag with the distinguishing arms of the colony at the centre. The Union Flag also appears in the canton (upper flagpole-side quarter) of the flags of several nations and territories that are former British possessions or dominions, as well as in the flag of the US State of Hawaii, which has no such connection.

The origins of the earlier flag of Great Britain date back to 1606. King 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 (X-shaped cross, or St Andrew's Cross) on a blue background, would be joined, forming the flag of England and Scotland for maritime purposes.

The present design of the Union Flag dates from a Royal proclamation following the union of Great Britain and Ireland in 1801. The flag combines aspects of three older national flags: the red cross of St George for the Kingdom of England, the white saltire of St Andrew for Scotland and the red saltire of St Patrick to represent Ireland. Although the Republic of Ireland is no longer part of the United Kingdom, Northern Ireland is.

There are no symbols representing Wales in the flag, making Wales the only home nation with no direct representation, as at the time of the Laws in Wales Acts 1535 and 1542 (creating legal union with England) the concept of national flags was in its infancy. The Welsh Dragon was however adopted as a supporter in the royal coat of arms of England used by the Tudor dynasty from 1485.