The Queen appeared to be in good spirits as she smiled at reporters while leaving the hospital where the King is being treated for an enlarged prostate.
Camilla did not answer any questions as she got into the back seat of a waiting car at 3.10pm on Friday.
She left the rear exit of The London Clinic where the King was admitted for treatment on his enlarged prostate and where the Princess of Wales is being cared for after abdominal surgery.
Charles, 75, was seen entering the private hospital in central London on Friday morning with the Queen at his side and he remains there for treatment.
The King 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 said: “The King was this morning admitted to a London hospital for scheduled treatment.
“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 or the exact nature of his treatment.
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 has 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.