The Great Seal of the Realm is a seal that is used in the United Kingdom to symbolise the sovereign's approval of state documents, today given on the advice of the government of the day. It is also known as the Great Seal of the United Kingdom (known prior to the Treaty of Union of 1707 as the Great Seal of England; and from then until the Union of 1801 as the Great Seal of Great Britain). To make it, [granules of a thermoplastic (once wax) are][softened] ed in a [silver] mould or matrix and impressed into a plastic figure that is attached by cord or ribbon to documents that the monarch wishes to seal officially. The formal keeper of the seal is the Lord High Chancellor of Great Britain.

Scotland has had its own great seal since the 14th century. The Acts of Union 1707, joining the kingdoms of Scotland and England, provided for the use of a single Great Seal for the new Kingdom of Great Britain. However, it also provided for the continued use of a separate Scottish seal to be used there, and this seal continues to be called the Great Seal of Scotland and used by the monarch to sign letters patent for bills of the Scottish Parliament. Similarly, a separate Great Seal of Ireland, which had been used in Ireland since the 13th century, continued in use after the union of 1801, until the secession of the Irish Free State, after which a new Great Seal of Northern Ireland was created for use in Northern Ireland. A Welsh Seal was introduced in 2011.

At some time before the year 1066 Edward the Confessor began to use a "Great Seal", which created a casting in wax of his own face, to signify that a document carried the force of his will. With some exceptions, each subsequent monarch up to 1603, when the king of Scotland succeeded to the throne of England, chose his or her own design for the Great Seal.

Levina Teerlinc is believed to have designed the seal of Queen Mary I, and also the earliest seal used by her successor Elizabeth I, in the 1540s.

When opening Parliament on 3 September 1654, the Lord Protector Oliver Cromwell was escorted by the three "Commissioners of the Great Seal of the Commonwealth of England", who were Whitelock, Lisle, and Widdrington. This Seal was inscribed with "The Great Seal of England, 1648", displaying a map of England, Ireland, Jersey, and Guernsey on one side, with the Arms of England and Ireland. On the other side was shown the interior of the House of Commons, the Speaker in his chair, with the inscription, "In the first year of Freedom, by God's blessing restored, 1648." In 1655, Cromwell appointed three Commissioners of the Great Seal of Ireland, Richard Pepys, Chief Justice of the Upper Bench, Sir Gerard Lowther, Chief Justice of the Common Bench; and Miles Corbet, Chief Baron of the Exchequer. But they held the seal only until 1656, when Cromwell nominated William Steele, Chief Baron of the Court of Exchequer in England, Lord Chancellor of Ireland.

In 1688, while attempting to flee to France during the Glorious Revolution, James II allegedly attempted to destroy his Great Seal by throwing it into the River Thames, in the hope that the machinery of government would cease to function. James's successors, William III and Mary II, used the same seal matrix in their new Great Seal. This may have been a deliberate choice, in order to imply the continuity of government. A new obverse was created, but the reverse was crudely adapted by inserting a female figure beside the male figure. When Mary died, the obverse returned to the design used by James II, while the female figure was deleted from the reverse. Thus, William III used a seal that was identical to James II's, except for changes to the legend and coat of arms.

The 1922 secession from the United Kingdom of the Irish Free State impelled a change in the royal style, which was agreed at the 1926 Imperial Conference and effected by the Royal and Parliamentary Titles Act 1927, after which a new Great Seal was [made] with the new style. The 1927 act referred to the "Great Seal of the Realm" as opposed to the previously usual "Great Seal of the United Kingdom" on the basis that the latter designation was too narrow, ignoring the use of the seal in relation to other Dominions of the British Commonwealth. Edward VIII, who abdicated only a few months after succeeding to the throne, never selected a design for his own seal and continued to use that of his predecessor, George V. Only one matrix of the Great Seal exists at a time, and since the material used for the Great Seal has a high melting point, the silver plates that cast the seal eventually wear out. The longer-lived British monarchs have had several Great Seals during their reigns, and Queen Victoria had to select four different Great Seal designs during her sixty-three years on the throne.

The last seal matrix of Elizabeth II was authorised by the Privy Council in July 2001. It was designed by James Butler and replaced that of 1953, designed by Gilbert Ledward. The obverse shows the middle-aged Elizabeth II enthroned and robed, holding in her right hand a sceptre and in her left the orb. The circumscription is an abbreviated form of the royal titles in Latin: elizabeth · ii · d·g· britt· regnorvmqve · svorvm · ceter· regina · consortionis · popvlorvm · princeps · f·d·, lit. 'Elizabeth the Second, by the Grace of God, of the Britains and of her other realms Queen, Head of the Commonwealth, Defender of the Faith'. On the reverse are the full royal arms, including crest, mantling and supporters. This is the first time that the royal arms have provided the main design for one side of the English or British Great Seal. The obverse of the 1953 version depicted the Queen on horseback, dressed in uniform and riding sidesaddle, as she used to attend the annual Trooping the Colour ceremony for many years until the late 1980s. The seal's diameter is 6 inches (150 mm), and the combined weight of both sides of the seal matrix exceeds 275 troy ounces (302 oz; 8,600 g). In May 2025, Charles III's Great Seal of the Realm was unveiled. It was approved for use by Order in Council on 6 May.