It is not prostate cancer, but was discovered during his recent treatment for an enlarged prostate.
The type of cancer has not been revealed, but the palace said the King began “regular treatments” on Monday.
Buckingham Palace says the King “remains wholly positive about his treatment”. He will postpone his public engagements, with senior royals expected to stand in for him.
The King “looks forward to returning to full public duty as soon as possible”, the palace said.
No further details are being shared on the stage of cancer or a prognosis.
The King informed both his sons personally about his diagnosis and Prince William was said to be in regular contact with his father.