Charles, 77, revealed the information in a recorded message broadcast on British television on Friday as part of a campaign to encourage people to take advantage of screening that can detect cancer in its early stages when it is easiest to treat.
"Early diagnosis quite simply saves lives," the king said.
"I know, too, what a difference it has made in my own case, enabling me to continue leading a full and active life even while undergoing treatment.
"Indeed, today I am able to share with you the good news that thanks to early diagnosis, effective intervention and adherence to 'doctors' orders', my own schedule of cancer treatment can be reduced in the new year.
"I know from my own experience that a cancer diagnosis can feel overwhelming.
"Yet I also know that early detection is the key that can transform treatment journeys, giving invaluable time to medical teams - and, to their patients, the precious gift of hope."
The recorded message gave Charles the opportunity to reflect on his experiences in the 22 months since he announced he would undergo treatment for an undisclosed type of cancer.
Charles' decision to disclose his diagnosis was a departure for Britain's royals, who have traditionally considered their health to be a personal matter and shared few details with the public.
"His majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer," Buckingham Palace said at the time.
Since then, Charles has used his own story to highlight the need for early diagnosis and treatment. Cancer Research UK recorded a 33 per cent increase in visits to its website in the weeks after the king's diagnosis, as people sought information about the signs of cancer.
While the palace hasn't specified what type of cancer the king has, officials said the cancer was discovered after treatment for an enlarged prostate revealed "a separate issue of concern".
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