Looking relaxed and smiling, the 75-year-old monarch said he was feeling "much better" while chatting and holding hands with people being treated at University College Hospital Macmillan Cancer Centre in London.
He was asked repeatedly how he was, replying: "I'm well".
Staff and patients alike said they were taken aback by how candid the King was and it was evident that he could relate to what they were going through.
Charles was joined by Queen Camilla, 76, on the visit and went on impromptu walkabouts, meeting workers who paused to catch a glimpse of the royal couple.
He also revealed that after his engagement, he would be having cancer treatment himself.
Speaking with Lesley Woodbridge, 63, as she underwent chemotherapy for sarcoma at the day unit, the King said: "It's always a bit of a shock, isn't it, when they tell you?"
Charles and Regis, Camilla had a long chat with Lesley, from Houghton Beds, who was joined by husband Roger, 64. He spoke to Camilla while Charles learned about his wife.
Roger said: "We were talking about how we feel. I said to her, 'How do you feel?' She just said, 'It's just so difficult' and we both agreed."
The King held Lesley's hand before he left and also held the hand of Asha Millan.
The pair appeared to share a joke, with Charles laughing heartily. Asha, 60, who is receiving chemotherapy for bone marrow cancer, said afterwards: "I said, 'How are you?' and he said, 'I'm well"."
The King has been receiving treatment for an undisclosed form of cancer since February and it was announced last Friday that he would be returning to public-facing duties. It marks a positive development after nearly three months as an outpatient.
Positive
When quizzed about his health, Charles told one patient: "I'm all right thank you very much, not too bad."
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