Kate held an audience with Olena Zelenska at the royal residence on Sunday afternoon, where they were pictured speaking.
Images capture the pair sitting side-by-side in front of a fireplace on gingham armchairs with purple orchids on display in the background.
Ms Zelenska earlier visited Westminster Hall to see the Queen lying in state.
No 10 had previously declined to confirm reports that the Ukrainian president’s wife would travel to the UK for the historic ceremony.
Ms Zelenska made the jouney to the UK after her husband, Volodymyr Zelensky, set time aside from leading his country against Russia’s war to sign a book of condolence for the late British monarch, who died last Thursday.
Responding the the visit of the top Ukrainian official, the UK’s ambassador to the war-torn eastern European nation said she was “deeply honoured” by the president’s gesture.
“Grateful to the president for taking the time to do this given all else that is happening in (Ukraine) at this time,” Melinda Simmons said.
The meeting of Kate and Ms Zelenska comes after Liz Truss’s earlier audience with the King in the Palace’s 1844 Room.
The prime minister has also continued informal talks with world leaders as hundreds of dignitaries descend on London for Monday’s funeral.
It comes after president Joe Biden and First Lady Jill Biden visited Queen Elizabeth II’s coffin as part of his trip to pay his respects to the late monarch.
Mr Biden arrived in London earlier this week to pay tribute to the Queen as part of a series of events.
Shortly afterward, Mr Biden also signed the condolence book for the Queen. They were then attending a reception at Buckingham Palace.
The Queen has been lying in state in London since Thursday evening, with huge queues of mourners lining up along the River Thames waiting upwards of 14 hours to visit Her Majesty’s coffin.
Mourners have been given until 6.30am on Monday to reach Westminster Hall in anticipation of tomorrow’s ceremony that will see the monarch transported to Westminster Abbey and finally to Windsor Castle.