King Charles attended church with his wife, Camilla, at St Mary Magdalene church in Sandringham, eastern England. Wearing a brown overcoat and carrying an umbrella, the king waved towards the crowd.
Earlier, Charles, who is spending time at his rural Sandringham estate, issued a message on Saturday expressing gratitude to well-wishers following his diagnosis.
While he has postponed some public engagements to undergo treatment, the king plans to continue with his private work as the monarch, including his weekly meetings with the prime minister and handling state papers.
The diagnosis of an unspecified form of cancer was announced by Buckingham Palace on Monday. King Charles, who ascended to the throne following the death of his mother of Queen Elizabeth, has been in power for less than 18 months.
The cancer was detected during a 3 day hospital stay last month when King Charles underwent a corrective procedure for a benign enlarged prostate. The palace has confirmed that the cancer is not prostate cancer but has not provided any further details.