Ukrainian prime minister Denys Shmyhal presented the King with a sculpture commemorating the liberation of Snake Island at a crowded reception in St James's Palace on the eve of the Ukraine Recovery Conference.
Mr Shmyhal described the bronze statue, depicting a soldier clutching a Ukrainian flag, as a symbol of his country's "mutual victory" with Britain over Russia.
The dramatic liberation, when Ukraine retook the small island after its defiant defenders were captured by Russian forces, is approaching its one-year anniversary at the end of June.
Charles held an audience with dozens of distinguished guests, including US secretary of state Antony Blinken, European Commission president Ursula von der Leyen, and Labour leader Sir Keir Starmer on Tuesday evening.
They arrived at the palace ahead of the two-day conference starting on Wednesday, where governments will meet to discuss rebuilding Ukraine after its devastating war with Russia.
The King was greeted in the gilt-edged throne room by Foreign Secretary James Cleverly who introduced him to Mr Shmyhal.
Presenting Charles with the sculpture, the prime minister said: "This is symbolic of our mutual victory.
"We are so grateful for the support of the UK - you are the number one supporter of Ukraine."
Ms von der Leyen was also in attendance alongside the president of Estonia, Alar Karis, and Latvian prime minister Arturs Krisjanis Karins.
Greeting the King, the EU chief said: "We are in this together."
At one point the King was asked how his horse performed at Royal Ascot - where the thoroughbred Saga was beaten by a rival called Royal Champion earlier on Tuesday - only for Mr Cleverly to interject, saying: "Oh! Wrong question."
It was not the only moment when the horse's poor performance became the subject of conversation.
While greeting Defence Secretary Ben Wallace later in the evening, the Conservative MP consoled the King, saying: "You were right in your prediction."
US President Joe Biden sent his "warmest regards" through Mr Blinken, the secretary of state told the King, adding: "And [he] also wants me to tell you how much he admires your leadership on the environment."
Before departing, the King also shook hands with Sir Keir and decorated Ukrainian veteran Masi Nayyem, who lost an eye when he was struck by shrapnel.
Mr Nayyem, who now campaigns to support veterans, said afterwards that Britain's support encouraged him to "fight until death" for his country.
He thanked the UK for providing "more than Europe", adding: "I fight for freedom - we are the last romantics of Europe.
"We believe our values and what we fight for."
The 2022 Ukraine Recovery Conference, held in Switzerland, established the Lugano Principles, which laid the groundwork for the reconstruction process.
It aims to galvanise international investment to rebuild the Ukrainian economy, making it a stronger and more stable democracy after the war.