King Charles I was presented with the new £5, £10, £20, and £50 banknotes featuring his portrait at Buckingham Palace.
The existing banknotes with Queen Elizabeth II's portrait will still be in circulation.
This tradition marks the first time the Bank of England has had to change the monarch on its banknotes.
King Charles, who has been participating in private events since his cancer diagnosis, met with Andrew Bailey, the governor of the Bank of England, for this occasion.
King Charles III was presented with new polymer banknotes featuring his image, which he described as "very elegant." The design does not include a crown, and the banknotes mark the final major step in the transition to his reign, following the release of new stamps and coins.
The Bank of England will gradually introduce the new notes, with Queen Elizabeth notes remaining in circulation until they are worn or damaged.
The new banknotes will be launched on June 5, but it could take some months before they become widely used.
In 2022 and 2023, various changes were made to British currency and documents to reflect the new monarch, King Charles III.
New 50p coins featuring his image were released in December 2022, along with other new coin designs.
The first King Charles stamps were issued by Royal Mail in March 2023, followed by new regular stamps without his crown.
British passports were changed to bear the title "His Majesty" instead of "Her Majesty" in July 2023.
Public buildings received an official portrait of the monarch taken at Windsor Castle in January 2024.
The transition to cashless payments may make these changes less noticeable at first.
In February 2024, King Charles's preferred design of the Tudor crown began appearing on the gov.uk website and official buildings.
In June 2024, the Bank of England issued new £5, £10, £20, and £50 banknotes featuring the King's portrait from a 2013 photograph.
However, postboxes, which are only changed when damaged or in need of extensive repair, are expected to be one of the last public symbols to reflect the change, as some still bear the insignia of Queen Victoria.