Prince Harry has spoken to the King about his cancer diagnosis and will travel to the UK to see him in the coming days.
The King has started treatment on Monday, a Buckingham Palace statement says.
The palace has not confirmed what type of cancer the 75-year-old monarch has.
The diagnosis comes following treatment for an enlarged prostate, although the palace says he does not have prostate cancer.
The King personally called both Harry and the Prince of Wales - as well as his siblings the Princess Royal, the Duke of York and the Duke of Edinburgh - to share news of his health, Buckingham Palace said.
It is not yet known whether Harry's wife, Meghan, or their two children, Prince Archie and Princess Lilibet, will come too.
The couple are yet to make a public statement.
Relations between the monarch and his younger son have been strained for some time.
The Duke of Sussex was critical of his family in his book, Spare, which was published last year.
Weeks after the publication, he was asked by the monarch to leave Frogmore Cottage, close to Windsor Castle, which had been their UK home.
It was seen as a major rebuke by the King to his son who, with the Duchess of Sussex, stepped down from royal duties and moved to the US in 2020, a decision said to have disappointed his father and Queen Elizabeth.
The Sussexes have also aired their grievances about their alleged treatment in a Netflix series.
In March, the couple were invited to the King's coronation, seen as the monarch offering an olive branch.
Harry attended, but the Duchess remained at home in California with the couple's children, four-year-old Archie and Lilibet, aged two.
In December, Buckingham Palace said it was considering whether to take action after the King and Princess of Wales were named as senior royals who questioned what skin colour Prince Archie would have.
The unsubstantiated allegation first surfaced in 2021, in the couple's interview with Oprah Winfrey, when Meghan alleged that a member of the Royal Family had raised "concerns" about Archie's skin colour before he was born.