The type of cancer has not been revealed, but according to a palace statement the King began "regular treatments" on Monday.
Buckingham Palace says the King "remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible".
He will postpone his public engagements and it is expected other senior royals will help to stand in for him during his treatment.
No further details are being shared on the stage of cancer or a prognosis.
The King, 75, returned to London from Sandringham in Norfolk on Monday morning and the palace says he has commenced treatment as an outpatient.
Although he will pause his public events, the King will continue with his constitutional role as head of state, including paperwork and private meetings.
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