Head-of-state Charles, 75, left the private London Clinic three days after undergoing prostate surgery.
The monarch, dressed in a suit and a black overcoat, waved to crowds as he and Queen Camilla, 76, emerged from the central London facility and got into a waiting car.
His departure came hours after Kensington Palace announced that his daughter-in-law Catherine - whose husband is heir to the throne Prince William - had also left the clinic after undergoing abdominal surgery earlier this month.
It is unclear when exactly the 42-year-old princess was discharged but her office said that she was making "good progress" at home at the start of a recuperation that could last months.
The pair's absence has created a frontline staffing shortage within the already slimmed-down monarchy, with William also postponing public engagements to be at his wife's side and Camilla left as the most visible working royal.
Charles, who became king upon the death of his mother, Queen Elizabeth II, on September 8, 2022, has generally enjoyed good health, barring injuries from polo and skiing.
The palace announced on January 17 that he would have a "corrective procedure" for the benign enlarged prostate the following week, and he was admitted to the clinic last Friday.
In an update, it said Charles, who spent three nights at the hospital, had rescheduled forthcoming engagements "for a period of private recuperation".
It gave no update on his medical condition, although he was said to have been "doing well" at the weekend.
"His Majesty would like to thank the medical team and all those involved in supporting his hospital visit, and is grateful for all the kind messages he has received in recent days," a statement read.
Charles wanted to publicise his own case to raise awareness about the condition, which is common in men aged over 50 and affects urination.
Symptoms include a frequent need to go to the toilet and difficulty in fully emptying the bladder.
The announcement has prompted a surge in internet searches for the term "enlarged prostate" on the state-run National Health Service (NHS) website.
Catherine, meanwhile, was treated at the hospital nearly two weeks ago and has now returned home to Windsor, west of London, to continue her recovery, according to the Kensington Palace statement.
Kate and William sent a "huge thank you" to hospital staff, and said the family "continues to be grateful for the well wishes they have received from around the world".
At the time of her operation, Kensington Palace stressed that her condition was not linked to cancer, without elaborating.
William, Charles's elder son, has postponed public engagements to be with his wife and to care for their three children, Prince George, 10, Princess Charlotte, eight, and Prince Louis, five, the palace said.
The surprise health threaten several overseas trips, according to media reports.
William and Kate had reportedly been set to travel to Rome in the coming months for their first joint overseas visit in two years.
There has also been speculation that Charles and Camilla would visit Australia this year.
Charles's sister, Princess Anne, 73, who recently went to Sri Lanka, is said to be ready to stand in for her brother at events in coming weeks.
The sudden shortage of senior working royals follows three years of upheaval.
Family patriarch Prince Philip died in 2021, then Queen Elizabeth II the following year.
Charles's younger son Prince Harry - fifth in line to the throne - and his wife Meghan quit their royal roles in early 2020 and relocated to California.
The elder of the king's two younger brothers, Prince Andrew, eighth in the line of succession behind William's three children and Harry's two, has also been sidelined.
That followed Andrew's disastrous handling of questions about his friendship with the convicted US sex offender Jeffrey Epstein, and his decision to settle a US civil claim for sexual assault without admitting liability.