The news comes after the King underwent treatment for an enlarged prostate in January.
During The King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted," the Palace noted in a statement shared on X, which is the social media platform formerly known as Twitter. "Subsequent diagnostic tests have identified a form of cancer."
The Palace did not say what type of cancer the King was diagnosed with but added he "is grateful to his medical team for their swift intervention, which was made possible than to his recent hospital procedure. He remains wholly positive about this treatment and looks forward to returning to full public duty as soon as possible."
The palace said the 75-year-old monarch has started a schedule of regular treatments, with doctors advising him to postpone public-facing duties.
Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual," according to the statement.
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," the Palace added.
U.K. political leaders sent messages of support.
“Wishing His Majesty a full and speedy recovery," Prime Minister Rishi Sunak posted on X. "I have no doubt he’ll be back to full strength in no time and I know the whole country will be wishing him well.”
Leaders in the U.S. also reacted to the diagnoses, with State Department Principal Deputy Spokesperson Vedant Patel sharing well wishes with the King and his loved ones.
"Our thoughts are with the King and his family," Patel said in an unrelated news conference on Monday.
King Charles III's diagnosis follows his daughter-in-law's own health issue, as Kate Middleton was released from the hospital at the end of January after a planned abdominal surgery. Kensington Palace said the Princess of Wales returned home to Windsor, where she is recovering.
"She is making good progress," according to a statement from Kensington Palace. "The Prince and Princess wish to say a huge thank you to the entire team at The London Clinic, especially the dedicated nursing staff, for the care they have provided."
King Charles III automatically became king upon death of his mother, Queen Elizabeth II, in 2022. She was 96 years old.
King Charles III was crowned in May, receiving the bejeweled St. Edward's Crown in a ceremony in England.
Trumpets sounded inside the medieval abbey and the congregation shouted “God save the king!” at a service attended by more than 2,000 guests, including world leaders, aristocrats and celebrities. Outside, thousands of troops, tens of thousands of spectators and a smattering of protesters converged.
The crowd of well-wishers swelled to hundreds of thousands by the time King Charles III and Queen Camilla emerged to wave from the Buckingham Palace balcony alongside younger generations of royals.
It was the culmination of a seven-decade journey for the king from heir to monarch.
King Charles III has two sons, including Prince William and Prince Harry. Prince William, who is King Charles III's oldest son, is next in line for the crown.
According to Page Six, Prince Harry is flying home to the UK to be reunited with his father following the diagnosis. The media outlet cited "a source close to the duke," adding that Prince Harry "will head to Windsor Castle in the next few days and has already spoken to the king" about the diagnosis.
"Harry’s wife, Meghan Markle, and their two children, Prince Archie and Princess Lilibet, will not be heading across the pond with him," Page Six reported.
EDITOR'S NOTE: The Associated Press contributed to this report.