The King is "doing well" after undergoing a procedure for an enlarged prostate.
The monarch, 75, arrived at the London Clinic hospital on Friday morning with the Queen at his side - the same hospital where the Princess of Wales is recovering after abdominal surgery.
The Queen left the London Clinic hospital just after 3pm on Friday - after which she returned for a visit lasting around an hour and a half before leaving the private hospital again just before 8pm.
She told people inside the hospital that her husband was "doing well".
Buckingham Palace confirmed that the King spent the night in hospital. Reports suggest he could remain there over the weekend.
In a statement earlier on Friday, the palace 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 understood that the King visited her ahead of his own treatment.
The King's public engagements will be postponed after he has his corrective procedure on an enlarged prostate.
It is understood he shared details of his diagnosis to encourage other men who may be experiencing symptoms to get checked in line with public health advice.
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 palace announced last week that the King, who arrived back in London from Sandringham in Norfolk on Thursday afternoon, would be having treatment less than two hours after it was announced that his daughter-in-law had undergone abdominal surgery and will remain in hospital for up to two weeks.
In a statement, the palace said: "In common with thousands of men each year, the King has sought treatment for an enlarged prostate.
"His Majesty's condition is benign and he will attend hospital next week for a corrective procedure.
"The King's public engagements will be postponed for a short period of recuperation."
The Queen said during an engagement in Aberdeen on 18 January that the King is "fine" ahead of his treatment and was "looking forward to getting back to work".
Sky News royal commentator Alistair Bruce said: "When the late Queen was in declining health it was felt that there was no need to go into great detail but I think that the King has been extremely imaginative in revealing the condition he faces and using it as a trigger for so many other people to consider their own health situation.
"So that is a change, but I think the palace will continue to keep the details of the process private."
The NHS describes a benign enlarged prostate as a condition that can affect how people urinate, and is common among men aged over 50.
"It's not a cancer and it's not usually a serious threat to health," the NHS said on its website.
"Many men worry that having an enlarged prostate means they have an increased risk of developing prostate cancer. This is not the case."
But benign prostate enlargement can sometimes lead to complications, such as a urinary tract infection, chronic urinary retention, and acute urinary retention.
The NHS also said the cause of prostate enlargement is unknown, but it is believed to be linked to hormonal changes as a man gets older.