The Queen visited The King at The London Clinic after he received treatment for an enlarged prostate.
The King is "doing well" as he spends his second day at the hospital after undergoing the procedure at the private facility on Friday morning.
Camilla arrived at a back entrance to the hospital - where the Princess of Wales is also being cared for following abdominal surgery - in a black Audi car at noon on Saturday to visit the King.
She left at 2:55pm in the same car.
On Friday, she left the hospital just after 3pm, after which she returned for a visit lasting around an hour and a half before leaving again just before 8pm.
She told people inside the hospital at the time that her husband was "doing well".
Ahead of his treatment, Charles visited his daughter-in-law Kate, who is on the 12th day of her hospital stay after undergoing a successful operation last week.
It is not known how long Charles will spend in hospital, but reports suggest he could remain there over the weekend.
The King arrived in London from Norfolk on Thursday afternoon ahead of the procedure after a couple of behind-the-scenes official duties.
He was diagnosed with the benign condition on 17 January after going for a check-up when he was experiencing symptoms.
It's understood he wanted to share the news to encourage others to get themselves checked.
NHS England reported a boost in views of its "enlarged prostate" page on the NHS website, which received one visit every five seconds on the day the diagnosis was announced.
A Buckingham Palace spokesman previously said the King was "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," the spokesman added.
Charles, who only acceded to the throne 16 months ago, cancelled engagements and was urged to rest by his doctors ahead of the corrective procedure.
The exact nature of the 75-year-old's treatment is not known.