The king was diagnosed after his recent hospital treatment for benign prostate enlargement.
He is beginning treatment today, the palace said.
In the meantime, the king will step back from ”public facing” duties.
“Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual,” the palace said.
“The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure.
King Charles waves after surgery
“He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.
“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.”
King Charles, 75, ascended to the throne after the death of his mother Queen Elizabeth II on September 8, 2022, and was crowned in May last year.