It was personally chosen by the King, from a range of designs produced by the College of Arms.
The monogram combines his initial "C" and "R" for Rex, the Latin for king, plus III for the third King Charles.
The cypher, a visual identity for the new reign, replaces the E II R of Queen Elizabeth II.
As the period of royal mourning ends, the new cypher of King Charles will be used for the first time.
Its first application will be in the Buckingham Palace post room on Tuesday, with the cypher used to frank letters from the Royal Households.
There is a separate version of the cypher for Scotland, which features the Scottish Crown.
But there won't be a sudden change on post boxes or on public buildings.
There are still post boxes in use from the reign of Queen Victoria, Edward VII, George V and VI, and the original cyphers remain until boxes need to be replaced.
Almost 70,000 of the current post boxes, about 60% of the total, date from the reign of Queen Elizabeth II. There are only about 170 surviving from the short reign of Edward VIII, who abdicated in 1936.
Boxes already under construction or ready for installation will continue to have the late Queen's cypher, Royal Mail says.
Where royal cyphers appear on buildings, it will be up to individual organisations to decide when or if they will be updated.