A new Great Seal, the fourth of George III's reign, was adopted on 1 January 1801 to coincide with the union of the United Kingdom and Ireland. On the obverse the enthroned King is attended by the allegorical figures of Hercules, Minerva and Justice to the left, and Britannia and Religion to the right. An equestrian portrait appears on the reverse, the King holding a baton, with a view of London and the River Thames in the background.

The Seal has fundamentally the same design as its immediate predecessor, introduced in 1785, the only significant changes being those made to the inscription and the Royal Arms. BRITANNIARUM was used in place of MAG BRIT FR ET HIB in the inscription, reflecting not only the union but also the abandonment of the title, King of France. The fleurs-de-lis of France were omitted from the Royal Arms while the Hanoverian devices were moved from the fourth quartering onto a small central shield.

Nathaniel Marchant had been appointed Chief Engraver of His Majesty's Seals in 1800 and he was most likely responsible for actually engraving the fourth Seal. Given that the design was essentially the same as the third Seal, however, the greater part of the credit should perhaps be given to Thomas Major, Chief Engraver of His Majesty's Seals for more than 30 years.