Camilla arrived at the London Clinic in central London in a black Audi car at noon on Saturday.
Wearing a white blouse and black jumper, she left around three hours later.
It is the same hospital where the Princess of Wales is being cared for following abdominal surgery.
The Queen’s visit on Saturday was her second to the private hospital since she arrived with Charles 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 took time to visit his daughter-in-law, Kate, who is on the 12th day of her hospital stay after undergoing a successful major operation last week.
A Buckingham Palace spokesman previously said the King 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 but reports suggest he could remain there over the weekend.
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.
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 only acceded to 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.
The Queen had previously said her husband is “fine” and looking forward to getting back to work.
News of his diagnosis came on the same day that Kensington Palace announced the princess was in hospital after undergoing abdominal surgery.
Kate, 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.