The flag of Great Britain, commonly known as the Union Jack or Union Flag, is a maritime flag of Great Britain that was used from 1606 to 1801. The design was ordered by King James VI and I to be used on ships on the high seas, and it subsequently came into use as a national flag following the Treaty of Union and Acts of Union 1707, gaining a regularized status as "the Ensign armorial of the Kingdom of Great Britain", the newly created state. It was later adopted by land forces, although the blue field used on land-based versions more closely resembled that of the blue of the flag of Scotland.

The flag consists of the red cross of Saint George, patron saint of England, superimposed on the Saltire of Saint Andrew, patron saint of Scotland. Its correct proportions are 1:2.

The flag's official use came to an end in 1801 with the creation of the United Kingdom of Great Britain and Ireland. At that time Saint Patrick's Flag was added to the flag of Great Britain to create the present-day Union Flag.