James Middleton reveals promise to Kate during her cancer treatment

James Middleton has revealed a promise he made to his sister Catherine, Princess of Wales following a major fundraising challenge for cancer research.

The 39-year-old entrepreneur paid tribute to Catherine, Princess of Wales after completing the National Three Peaks Challenge for The Royal Marsden Cancer Charity. He also recalled a promise he made to her during her cancer journey.

Taking to Instagram on Sunday, James shared that he had promised his sister during her treatment that they would one day complete the hiking challenge together.

"I'm so incredibly proud of my darling sister," he wrote alongside a photograph of him embracing Catherine, as well as another image showing the siblings with their parents Carole and Michael Middleton, Catherine's husband Prince William, and their three children, Prince George, 12, Princess Charlotte, 11, and Prince Louis, eight.

He went on to explain that the challenge came two years after he first made the pledge.

"Two years ago, I told you we'd climb this mountain together. We talked about climbing mountains while you were in hospital, and about the incredible healing power that nature can have for both the body and the mind," he wrote. "So it was such an honour to join you for your Three Peaks Challenge for the Royal Marsden Cancer Charity."

James also praised the Princess of Wales for her strength throughout her health battle.

"Seeing how far you've come is nothing short of inspiring," he penned. "Your strength, resilience, and determination, all while being the most amazing mother, wife, daughter, and sister, inspire me and so many others every single day."

He concluded the heartfelt message, "Keep being exactly who you are."

In late June, Catherine completed the Three Peaks Challenge to raise funds for The Royal Marsden, where she received treatment. The expedition involved climbing Ben Nevis, Scafell Pike and Snowdon, covering around 23 miles in total.

The royal first shared her cancer diagnosis in March 2024. In January 2025, she announced that she was in remission following her treatment.