Heir to the throne William will be joined by his aunt the Princess Royal, who is attending in a private and personal capacity as godmother of the new king, Haakon VIII, Buckingham Palace announced.
Norway has seen an outpouring of grief for the much-loved monarch, who died on August 28 aged 89.
Known for his warmth, humour and informal style, Harald has been hailed as an inclusive "people's king" who modernised the Norwegian monarchy during his 35-year reign.
His funeral will take place on Wednesday at Oslo Cathedral, with mourners including his widow Queen Sonja.
In keeping with precedent that the sovereign does not attend funerals of other heads of state, the duty falls to William, a future king, to represent his father and pay his respects at the poignant service.
Charles paid a touching tribute to his "dear cousin" and "much-loved kinsman and friend" after the news of Harald's death was announced.
In a message of condolence, he praised Harald's "unwavering dedication" and his "kindness, compassion and deep sense of duty echoed from across the sea and across the decades".
The late Queen Elizabeth II and King Harald were second cousins. They shared the same great-grandparents, King Edward VII and Queen Alexandra.
In April 2025, William represented his father at Pope Francis's funeral in the Vatican.
William and the now-Princess of Wales conducted an official tour to Norway in 2018, dining with King Harald and Queen Sonja at the Royal Palace, where Harald described them as "family" and praised their support of the late Queen.
The Norwegian sovereign processed into the banquet hall arm-in-arm with pregnant Kate, who was expecting Prince Louis and wearing a flowing Alexander McQueen gown, with the gesture mirrored by his wife Queen Sonja and William, who walked behind them.
William and Kate also spent time visiting the ski slopes with the then-Crown Prince Haakon and Crown Princess Mette-Marit, now Norway's new King and Queen.
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