The national flag of the United Kingdom is the Union Jack, also known as the Union Flag.

The design of the Union Jack dates back to the Act of Union 1801 which united the Kingdom of Great Britain and the Kingdom of Ireland (previously in personal union) to create the United Kingdom of Great Britain and Ireland. The flag consists of the red cross of Saint George (patron saint of England), edged in white, superimposed on the saltire of St Patrick (patron saint of Ireland), also edged in white, which are superimposed on the saltire of Saint Andrew (patron saint of Scotland). Wales is not represented in the Union Flag by Wales's patron saint, Saint David, because the flag was designed while Wales was part of the Kingdom of England.

The flag's standard height-to-length proportions are 1:2. The war flag variant used by the British Army has proportions 3:5.

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 Flag shall be azure, the Crosses saltire of Saint Andrew and Saint Patrick quarterly per saltire, counter-changed, argent and gules, the latter fimbriated of the second, surmounted by the Cross of Saint George of the third fimbriated as the saltire.