The King issued a message to US President Donald Trump on a "truly historic milestone" as he marked the 250th anniversary of America's independence from Britain on Saturday.
It comes just three months after Charles and his wife, Queen Camilla, visited the US, with the monarch adding that they were "proud, and honoured" to celebrate the special anniversary.
In his message shared on social media, the King said that the story of the relationship between the two nations is "one of extraordinary evolution, from overcoming conflict to forging one of the closest and most productive alliances the world has seen".
The monarch also said "the connection between our peoples is one that I trust will only grow stronger with time" and ended the message "Charles R".
© Getty ImagesTheir four-day trip in April saw them taking tea at the White House with Mr Trump and his wife Melania, attending a white-tie banquet in their honour, the King's historic address to Congress and a bunting-decked street party in rural Virginia.
It followed the President's second historic state visit to the UK last September, where he was feted with a grand state dinner at Windsor Castle.
The King's message comes after he and the Queen marked Royal Week or Holyrood Week in Scotland, where the monarch traditionally spends the time at the Palace of Holyroodhouse in Edinburgh.
The King's message to the President in full
"My wife and I are delighted to send our warmest congratulations to you and to the people of the United States of America on the occasion of the 250th anniversary of your nation’s independence.
"We were proud, and honoured, to be able to celebrate this special anniversary during our visit to the United States in April.
"This year marks a truly historic milestone and a moment of great celebration for Americans everywhere.
"It offers an opportunity to pause and reflect on the remarkable journey of the United States over the past two-and-a-half centuries, and to honour all that has been achieved since 1776.
© Getty Images"The story of the relationship between our two nations is one of extraordinary evolution, from overcoming conflict to forging one of the closest and most productive alliances the world has seen.
"Across defence and security, trade and investment, science, research, education, culture and the arts, the ties between the United Kingdom and the United States are unique and far-reaching.
"Together, we face the challenges of today, and the opportunities of tomorrow. As I said in my address to the joint Houses of Congress, our shared roots run deep, and they are still vital.
"I am greatly encouraged by the enduring bond between our countries, and by the many connections between our peoples, which we were able to see once again, at first-hand, on our visit.
© Alamy Stock Photo"Through centuries of shared challenge and achievement, our nations have built a relationship founded on friendship, trust and a belief in liberty, the rule of law and the dignity of all people.
"As we look ahead to the next 250 years, I have no doubt we will continue to defend our shared values.
"I also hold a deep appreciation of our shared responsibility to protect the natural world, knowing the decisions we take today will determine whether our critical natural systems will not just maintain the harmony and essential diversity of nature, but also continue to provide the foundation for our prosperity and our national security for both present and future generations.
"The connection between our peoples is one that I trust will only grow stronger with time.
"As you mark this momentous occasion, my wife and I send our warmest good wishes to you and to the people of the United States of America, today and in the years ahead. Charles R."
