Catherine, Princess of Wales visited the hospital where she was treated for cancer, after doing a charity climb earlier this year to help fund its new unit.
"This was my small way of being able to give back and say thank you," Catherine told staff at the Royal Marsden in London on Friday.
Middleton revealed she was having cancer treatment in 2024, and said she was in remission in January 2025.
In June this year, the princess carried out the Three Peaks Challenge to help the hospital with funding, which involved climbing the highest peaks of England, Scotland and Wales in just 24 hours.
The money raised, which is not public, will now be used for the The Centre for Holistic Wellbeing and Recovery, a unit offering complementary healing alongside clinical care.
Middleton spoke to patients at the hospital about the importance of support for the whole person.
"I had incredible care here and support from a huge team and I'm really, really grateful on a personal level," she said.
"But also the holistic care, the treatments and therapeutic support on the outside is so important.
"It makes a big difference - the impact of nature, the impact of good nutrition, good ongoing support around you when it's really difficult to manage the trials and tribulations of medical treatments."
A patient told her, "When you first get cancer, you're very aware of the physical impact. The mental impact comes after."
The princess agreed, saying its "not just the physical changes your body is going through.
"There's a mind-body aspect that changes who you are... having professionals who can help you navigate that is really essential," she said.
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