The King met with US President Joe Biden for tea and climate discussions at Windsor Castle on Monday.
Mr Biden, 80, was greeted with a Guard of Honour as he arrived to meet King Charles, 74, in the Quadrangle of Windsor Castle.
During the ceremony, the Prince of Wales's Company of the Welsh Guards gave a Royal Salute, while the US National Anthem was played by the Band of Welsh Guards.
The President, dressed in a navy suit, inspected the Guard of Honour while accompanied by the King, before the pair watched the military march past. See the moment here...
The pair sat down for tea in Windsor Castle, before His Majesty showed the US leader an exhibition in the White Drawing Room of items from the Royal Collection relating to the United States of America.
In the Green Drawing Room, the King and Mr Biden then joined participants of the Climate Finance Mobilisation Forum, which took place earlier in the day.
The Forum brought together finance and philanthropic leaders from the United Kingdom and the United States of America to strengthen their commitments to climate action within emerging markets and developing economies.
Monday marked the first time the pair have met since the coronation as First Lady Jill Biden flew over to represent the US.
Mr Biden was in attendance at the Heads of State Reception at Buckingham Palace on the eve of the late Queen Elizabeth II's funeral before he and Mrs Biden attended the funeral service in September last year.
They had previously met at the G7 Leaders Summit in June 2021 and Cop26 in Glasgow later that year.
They shared a handshake at Cop26 and Mr Biden expressed his admiration for Charles' passion for combating climate change and his environmental activism, telling him: "We need you badly."
"You are very kind for saying that," Charles replied.
Why did President Biden meet King Charles?
The UK visit is a layover before Mr Biden heads to the Nato summit in Lithuania.
Earlier in the day, the US leader sat down for a meeting with Prime Minister, Rishi Sunak at Downing Street.
Why didn't President Biden attend King Charles's coronation?
Mr Biden's decision not to attend Charles and Camilla's coronation at Westminster Abbey on 6 May is believed to be a result of his desire not to break a tradition spanning from the very birth of the United States of America.
This tradition dictates that US Presidents should not attend events such as coronations as a result of the USA's proud independence from the rule of the United Kingdom.