Charles, 75, and his wife Queen Camilla flew by helicopter to Buckingham Palace from the royals’ Sandringham estate in Norfolk, eastern England.
The monarch was seen waving to well-wishers as the couple were driven from the palace to nearby Clarence House.
Royal officials announced last week that the king was diagnosed with cancer and will suspend public engagements, though he will continue with state business such as signing papers.
King Charles III and Queen Camilla arriving back at Clarence House on Feb. 13 in London after spending a week at Sandringham in Norfolk, following the announcement of the king's cancer diagnosis.
They did not disclose what form of cancer he has, but said it’s not related to his recent treatment for a benign prostate condition.
The king underwent his first bout of cancer treatment last week. He attended church at Sandringham on Sunday, and thanked the public for the many messages of support he has received.
Camilla, who has been continuing with her royal diary of engagements, said last week that Charles was “extremely well under the circumstances.”