The King is “doing well” after undergoing a procedure for an enlarged prostate, sources said.
Charles was seen entering The London Clinic on Friday morning with the Queen at his side – the same hospital where the Princess of Wales is being cared for after abdominal surgery.
Camilla left the private hospital at 3.10pm on the same day and appeared to be in good spirits as she smiled at reporters before getting into a car.
It is understood the King’s scheduled treatment has now taken place and he is doing well.
Earlier on Friday, Charles took time to visit his daughter-in-law Kate, who is on the 11th day of her hospital stay after undergoing a successful major operation last week.
A Buckingham Palace spokesman previously said Charles was “admitted to a London hospital for scheduled treatment”.
The spokesman added: “His Majesty would like to thank all those who have sent their good wishes over the past week and is delighted to learn that his diagnosis is having a positive impact on public health awareness.”
It is not known how long Charles will spend in hospital.
The King arrived back in London from Norfolk on Thursday afternoon, ready for the procedure after squeezing in a couple of behind-the-scenes official duties.
Prime Minister Rishi Sunak wished Charles the “very best” for his treatment and a “speedy recovery” afterwards, a Downing Street spokeswoman said.
He was diagnosed with the benign condition on January 17 while staying at Birkhall, Aberdeenshire, after going for a check-up because he was experiencing symptoms.
The King is understood to have wanted to share the news to encourage other men to get themselves checked.
Charles, who only acceded to throne 16 months ago, cancelled engagements and was urged to rest by his doctors ahead of the corrective procedure.
The Queen had previously said her husband is “fine” and looking forward to getting back to work.