The King wore a new tartan for his visit to the Braemar Royal Highland Gathering, a short distance from his summer residence at Balmoral in Aberdeenshire.
The King Charles III tartan, named after the monarch, is green, blue and red and was designed by the Scottish Tartans Authority earlier this year to mark the coronation.
The King was accompanied by the Queen, wearing an outfit by Mr Roy, with his sister the Princess Royal and her husband Vice Admiral Sir Tim Laurence.
John McLeish, chair of the Scottish Tartans Authority, said: “Our iconic national cloth has a distinct style and evokes deep feelings of belonging, making it the best-known and best-loved fabric in the world.
“The King is an ambassador for tartan, Highland Dress, and the traditions of the Highlands.
“We are honoured that His Majesty has agreed that a new tartan should be created in his name.
“The King Charles III tartan now takes its place in the history books. It was a great privilege to be at Braemar today to witness His Majesty wearing the tartan for the first time.”
Traditionally, the monarch becomes chief of the gathering, although Charles has not yet been announced as such.
It is the first time that the King has appeared as monarch at the games, however, he frequently attended while he was still the Prince of Wales.
The event is a favourite with the royals, and the family used a photograph taken at the games last year on one of their Christmas cards.
The King handed trophies to winning competitors, who took part in a number of sports, including the shot put and the tug-of-war.