Camilla was seen entering a back entrance of the London Clinic in central London on Saturday at noon, at the same hospital where the Princess of Wales is being cared for following abdominal surgery. The Queen left around three hours later.
The visit on Saturday was her second to the private hospital since she arrived with the King on Friday morning ahead of the procedure.
She told people inside the London Clinic on Friday that the King was “doing well” after the procedure.
Charles, 75, took time to visit his daughter-in-law Catherine, who is on the 12th day of her hospital stay after undergoing a successful major operation last week.
The King arrived in London from Norfolk on Thursday afternoon, ready for the procedure after squeezing in a couple of behind-the-scenes official duties.
Rishi Sunak, Britain’s Prime Minister, wished the King the “very best” for his treatment and a “speedy recovery” afterwards, a Downing Street spokeswoman said.
The monarch 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 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.
NHS England said the “enlarged prostate” page on the NHS website received one visit every five seconds on the day the King’s diagnosis was announced, with further huge boosts in visits in the days that followed.
News of his diagnosis came on the same day that Kensington Palace announced the princess was in hospital after undergoing abdominal surgery.
Catherine, 42, is not expected to carry out official engagements until after Easter, with the Prince of Wales clearing his diary of official duties for the time being.