The King welcomed a special guest to Windsor Castle on Bank Holiday Monday.
The head of the Luxembourg royal family, Grand Duke Henri, attended a dinner at the British monarch's Berkshire residence.
A photograph shared on the Grand Ducal Court's Instagram account showed King Charles, 75, and Grand Duke Henri, 69, both dressed smartly in suits and standing in the Grand Corridor at Windsor Castle.
An array of ornate paintings, statues and furniture can be seen in the background, along with its gilded ceiling and domed window.
The caption read: "During a short stay in England, HRH the Grand Duke was received on Monday by HM King Charles III for a dinner at Windsor Castle."
Charles and Henri are distant cousins related through King George II and Queen Caroline, who ruled in Britain in the mid-1700s.
Grand Duke Henri and his wife, Grand Duchess Maria Teresa, both attended King Charles' coronation at Westminster Abbey in 2023.
The royal couple recently welcomed their eighth grandchild following the birth of their daughter Princess Alexandra's first child.
The baby girl – named Victoire – was born in Paris on 14 May, but the Grand Ducal Court is yet to share the first photograph of the tot.
King Charles' calendar
The monarch issued a personal message on Monday as he shared his and wife Queen Camilla's "heartfelt condolences to the families and communities" affected by the landslide in Enga in Papua New Guinea.
The King was also presented with the first seedling grown from seeds collected from the Sycamore Gap tree, nearly eight months after the historic tree was felled in an act of vandalism.
The much-photographed tree, which stood next to Hadrian’s Wall in Northumberland for 200 years, was chopped down in September last year, causing a national outrage.
Once the seedling has matured into a sapling, Charles will have it planted in Windsor Great Park for visitors to enjoy as a symbol of the hope and beauty that can come from loss, the National Trust said on Monday.
And on Tuesday, the King held an audience with the President of the Republic of Latvia at Buckingham Palace.
The palace has previously stated that future royal engagements "which may appear to divert attention or distract from the election campaign" may be postponed in light of the upcoming General Election in the UK on 4 July.