The new King debuted his new signature while signing the proclamation of his reign, signing the official documents "Charles R".
The 'R' stands for rex, Latin for 'king'. Queen Elizabeth II's signature also included an 'R' after her name but stood for Regina, which means 'queen' in Latin.
Queen Consort Camilla also has added an 'R' to her signature on the proclamation.
William, the Prince of Wales, signed the proclamation as 'William P,', with the 'P' standing for Prince.
King Charles III was officially announced as King two days following the death of his mother, Queen Elizabeth II.
Charles automatically became King when his mother died on Thursday, but the accession ceremony was a key constitutional and ceremonial step in introducing the new monarch to the country, a relic of a time before mass communications.
On Sunday, the Governor-General of Australia, David Hurley, officially proclaimed King Charles III as Australia's new sovereign during a ceremony at Parliament House.
With AAP.