The King is following in the footsteps of his late mother in becoming only the second British monarch in history to address Congress.
He received a four-minute standing ovation as he walked into the packed chamber before he had even said a word, and another loud round of applause as he took to his feet to speak.
The King expressed his 'particular gratitude to you all for the great honour of addressing this Joint Meeting of Congress'.
In the packed chamber, he thanked the American people 'on behalf of the Queen and myself' for welcoming them to the US to mark the semi-quincentennial year of the Declaration of Independence.
He also condemned the attempted assassination of US President Donald Trump at a Washington media dinner on Saturday saying that 'such acts of violence will never succeed'.
The King sparked laughter in the chamber when he asked for volunteers to adopt the role of the MP who is ceremonially held 'hostage' at Buckingham Palace while the King delivers his speech at the State Opening of Parliament.
'These days, we look after our guest rather well – to the point that they often do not want to leave. I don't know, Mr Speaker, if there were any volunteers for that role here today…?' the King said.
Quoting the playwright Oscar Wilde, Charles remarked: 'And for all of that time, our destinies as nations have been interlinked.
'As Oscar Wilde said, 'We have really everything in common with America nowadays except, of course, language'.'
He will emulate the late Queen's legendary brand of soft diplomacy by using his speech to calm troubled waters, telling politicians: 'Time and again, our two countries have always found ways to come together.'
King Charles addressing Congress, following in the footsteps of his late mother in becoming only the second British monarch in history to address Congress
King Charles III greets Vice President JD Vance and House Speaker Mike Johnson, as he arrives to speak to a joint meeting of Congress
Quoting the playwright Oscar Wilde, Charles remarked: 'And for all of that time, our destinies as nations have been interlinked'
He received a four-minute standing ovation as he walked into the packed chamber before he had even said a word
King Charles and US Speaker of the House Mike Johnson stand for photos before a Joint Meeting of Congress at the US Capitol
He is also set to make a pointed rejoinder to President Donald Trump's scathing dismissal of the UK's armed forces, particularly the Royal Navy, gently pointing out his own proud history in the service.
In one of his recent broadsides, sparked by what he sees as Prime Minister Sir Keir Starmer's failure to support his war with Iran, the American leader described UK assets as 'toys', adding: 'You don't even have a navy. You're too old and had aircraft carriers that don't work.'
Royal aides say His Majesty - who is head of the British armed forces - will reference 'with pride' his own service in the Royal Navy, and emphasise how the US and UK have stood together through moments that have defined the countries' shared history, observing how 'our defence, intelligence and security ties are measured not in years but in decades'.
Queen Elizabeth II addressed the joint meeting of Congress in the Capitol a quarter of a century ago, when she visited the US in 1991.
The King began his visit by meeting the Speaker of the House of Representatives, Mike Johnson.
This was followed by a meeting with the four leaders of the US Congress: Speaker Mr Johnson; House Democratic Leader Hakeem Jeffries; Senate Majority Leader John Thune; and Senate Democratic Leader Chuck Schumer.
The Escorting Committee, a bipartisan group of members of Congress appointed by the four leaders, preceded their Majesties into the chamber.
The House Sergeant at Arms and the Senate Sergeant at Arms then announced the King and Queen's arrival at the doors of the chamber.
King Charles and US House Speaker Mike Johnson (R-LA) walk through Statuary Hall
(left to right) Senate Majority Leader John Thune, Senate Minority Leader Chuck Schumer, Democratic Leader Rep. Hakeem Jeffries, King Charles III and Rep. Mike Johnson, Speaker of the House of Representatives, during the King's meeting with congressional leaders
US Speaker of the House Mike Johnson arrives for Britain's King Charles' address during day two of the State Visit
US Vice President JD Vance and Speaker of the House Mike Johnson (R-LA) stand before the start of a joint meeting of Congress with King Charles
Vice President JD Vance, Speaker of the House Mike Johnson (R-LA) and members of Congress wait for the arrival of King Charles III
His Majesty was greeted at the podium by the Speaker of the House and the Vice President of the US, before addressing the chamber.
His speech will not shy away from referencing current topical matters including Nato - which Mr Trump has threatened to pull out of - Aukus, the Middle East and Ukraine.
His 20-minute address is written on the advice of Government to support their strategic priorities, but the language and tone will be his own.
The overall message of the speech will be that while we live in times of great challenge, nationally and internationally, by defending and reaffirming our shared democratic values - values which have been woven deep into the fabric of both our nations throughout history - we can promote security and prosperity now and for the future, not just for our two countries, but for the whole world.
In his opening remarks, Charles, 77, will also make brief reference to the events of Saturday evening - when a shooter attempted to assassinate him and senior members of his administration at the White House Correspondents' Dinner - in a spirit of 'solidarity, sympathy and support'.
The King will tell his audience that he brings 'the highest regard and friendship of the British people to the people of the United States' in the landmark anniversary year, when the country is marking the 250th anniversary of the Declaration of Independence.
Turning to his main theme, he will reflect that while the UK and US have not always agreed on all matters over the past 250 years, the foundations of our 'democratic, legal and social traditions' - stretching all the way back to the Magna Carta - are such that 'time and again, our two countries have always found ways to come together'.
When they do so, he will say, the results are not just of national, but global benefit.
British Ambassador to the U.S. Christian Turner, Yvette Cooper, Secretary of State for Foreign, Commonwealth and Development Affairs, King Charles III, Speaker of the House Mike Johnson (R-LA), Senate Majority Leader John Thune (R-SD) and U.S. Senate Minority Leader Chuck Schumer (D-NY) attend a bicameral, bipartisan leadership meeting
Speaker of the House Mike Johnson, R-La., left and Britain's King Charles III walk through Statuary Hall
King Charles stands smiling with President Donald Trump during the arrival ceremony on the South Lawn of the White House in Washington
King Charles and Mike Johnson walk through Statuary Hall before a joint meeting of Congress in the House Chamber of the US Capitol
UK Foreign Secretary Yvette Cooper (left), King Charles (centre) and US Speaker of the House Mike Johnson
In addressing these values, the King will speak of his own faith and his belief that in the hearts of our two nations lies 'a generosity of spirit and a duty to foster compassion, to promote peace, to deepen mutual understanding and to value people of all faiths and none'.
Such values, he will say, need defending.
He plans to reflect on how defending our common ideals is 'crucial for liberty and equality', as is the rule of law which lies at the heart of our democratic traditions and of our shared economic prosperity.
Looking to the future, His Majesty will reference the importance of the trade and technology deals that will play a vital part the continued economic partnership of the two counties.
He will also celebrate America's many natural wonders, and stress how important it is to defend and protect 'Nature's own economy', too.
How well those comments will sit with President Trump, a notable climate change denier who has ripped up many of his country's green initiatives, remains to be seen.
However the two men have cordially discussed the issue before.
The King will conclude by noting that the story of our two nations over the last 250 years is one of 'reconciliation and renewal', giving rise to 'one of the greatest alliances in human history'.
It is an alliance which His Majesty hopes and prays will continue to protect and celebrate the values he holds dear, long into the future.
Later in the day the King will give another speech at a glittering state dinner at the White House, where President Trump will also speak.
It came after a successful day for the King after the American leader spoke passionately about the bonds between the two nations as he staged a ceremonial military welcome for Charles and Queen Camilla at the White House.
Despite the recent tensions between the UK and the US, Mr Trump, speaking from a podium on the South Lawn on the second day of the King's historic state visit, declared: 'In the centuries since we won our independence, Americans have had no closer friends than the British.'
And he paid tribute to the 'essence' of the 'special relationship' in the wake of the Second World War , saying: 'That understanding of our nation's unique bond and role in history is the essence of our special relationship and we hope it will always remain that way.'
In a speech Mr Trump branded the US and the UK 'the two most exceptional nations the world has ever known', calling on the nations to go forward 'together... with even stronger resolve'.
Mr Trump concluded: 'Your Majesties, thank you once again for making this important visit. We are so honoured.'