The King and Queen said they had left a piece of their heart in the US as they ended their State Visit on Thursday.
In an official message King Charles and Queen Camilla thanked Americans for 'the warmth of your welcome and the kind support you gave us' during their four-day trip, which coincided with the 250th anniversary of the signing of the Declaration of Independence.
Their Majesties said they had left a piece of the heart behind and taken 'a little of yours back home with us' following the visit, during which the King met with President Donald Trump and gave an historic address to the US Congress.
The message ended: 'Until the next time… God Bless America.'
As the Royal couple left the US Mr Trump said he would remove all tariffs and restrictions on whiskey imports in honour of their visit.
The president said the couple 'got me to do something that nobody else was able to do, without hardly even asking'.
In response a Buckingham Palace spokesperson said the King sent his 'sincere gratitude' to Mr Trump and said he would 'be raising a dram to the President's thoughtfulness'.
The Royal Family's official X account posted a farewell message, which included an image of the royal couple at Blair House, the president's guest lodging opposite the White House, where they stayed during the visit.
The King and Queen posted a farewell message at the close of their State Visit to the US, sharing an image of them together at Blair House, where they stayed during the trip
Their Majesties also shared a photo of the New York skyline, which featured the JPMorgan Chase building lit with a Union flag and the Empire State Building in red, white and blue on Tuesday
Charles and Camilla were photographed looking at each other in the Jackson Place sitting room, dressed in blue and white respectively.
In another image which accompanied the goodbye message, a Union flag was emblazoned on the summit of the JPMorgan Chase building in New York, pictured alongside the Statue of Liberty and the Empire State Building, which was lit in red, white and blue.
The full message said: 'Farewell and thank you for the warmth of your welcome and the kind support you gave us throughout our first visit to the US as King and Queen, in this, your special anniversary year.
'We leave a piece of our [heart emoji] behind and take a little of yours back home with us. Until the next time… God Bless America.'
It was signed: 'Charles R. & Camilla R.'
The King and Queen's visit - which saw them travel to Washington DC, New York and Virginia - has been seen as a success by commentators, who praised Charles's speech to Congress and the lifting of whiskey tariffs.
Charles and Camilla posted a farewell message at the close of their triumphant four-day State Visit
As the visit ended, US President Donald Trump announced he would remove all tariffs on whiskey in honour of the King and Queen
It had initially been seen as a risk, taking place amid the US-led war in Iran and after Mr Trump's strong criticism of Nato - he said the military alliance 'wasn't there when we needed them'.
But on Thursday the president hailed Charles as 'the greatest King' and reportedly called the Royal couple 'great people'.
Mail columnist Robert Hardman wrote this week that 'for a few days - at least - the relationship is going to be as "special" as it has ever been'.
At the end of the US visit, the King flew to Bermuda for a solo three-day tour.
Arriving at LF Wade International Airport at dusk, the UK Government's Airbus flew the Royal Standard and Bermuda flag from the cockpit window.
Charles was greeted by Governor of Bermuda Andrew Murdoch, its Premier David Burt and his wife Kristin - a small guard of honour formed by the Royal Bermuda Regiment stood to attention nearby.
The King is set to receive a ceremonial welcome of pomp and pageantry featuring a 21-gun salute on Friday.