The form of cancer has not been divulged, although the palace stated that the King started "regular treatments" on Monday.
According to Buckingham Palace, the King "remains wholly positive about his treatment". He will postpone his public appearances, with senior royals anticipated to fill in for him.
The palace stated that the King "looks forward to returning to full public duty as soon as possible".
There are no further data about the cancer's stage or prognosis.
The King personally informed both of his sons of his diagnosis, and Prince William was thought to maintain regular communication with his father.
Prince Harry, who lives in the United States, spoke with his father and will come to the UK to meet him in the coming days.
The King, 75, returned to London from Sandringham in Norfolk on Monday morning, and the palace reports that he is now receiving outpatient treatment.
Although he will suspend his public appearances, the King will continue to fulfill his constitutional job as head of state, which includes paperwork and private meetings.
When the head of state is unable to perform official duties, a constitutional process allows "counsellors of state" to be appointed to act in place of the king.
At the moment, that includes Queen Camilla, Prince William, Princess Anne, and Prince Edward, with Prince Harry and Prince Andrew no longer serving as non-working royals.
Prince William had also taken a break from public appearances to assist his wife Catherine, the Princess of Wales, with her recovery following "abdominal surgery".
However, it was reported earlier on Monday that he would return to public activities later this week.
The King was spotted during a church service in Sandringham on Sunday, waving to the throng.
He underwent a prostate operation at a private London hospital over a week ago.
The King chose to go public with his prostate treatment in order to encourage more men to have their prostates checked, according to the palace.
He was stated to be overjoyed to have raised awareness about the issue, with the NHS website noting an increase in inquiries concerning prostate diseases.
Many types of cancer are more likely to develop as people age. According to UK statistics, more than a third (36%) of new cancer cases are diagnosed in adults aged 75 and up.
Prime Minister Rishi Sunak, Labour leader Sir Keir Starmer, and Commons Speaker Sir Lyndsay Hoyle all wished the King a "full and speedy recovery".
Palace statement in full
