The king was pictured arriving with his wife Queen Camilla at the private London Clinic in west London, where Kate, the Princess of Wales, is also having treatment after undergoing abdominal surgery last week.
A royal source said Charles had visited Kate ahead of his own treatment.
"The king was this morning admitted to a London hospital for scheduled treatment," Buckingham Palace said in a statement.
"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."
Last week the palace that Charles, 75, would undergo a "corrective procedure" for a benign condition which was common among men over 50, and Camilla said her husband was "fine" and "looking forward to getting back to work".
The palace has declined to say how long the king would stay in hospital but his upcoming public engagements have been postponed to allow a short period of recuperation.
Britain's royals usually do not disclose details of illnesses, regarding all medical issues as a private matter, but Charles was keen to share details of his condition to encourage other men experiencing symptoms to have a medical check.
The state-run National Health Service said there had been a 1,000% increase in visits to its webpage giving advice on prostate enlargement since Charles' diagnosis was revealed.
His treatment is one of a series of health blows for the royals over the past week.
Kate, 42, is recovering in hospital after undergoing surgery for a non-specified, but non-cancerous, condition.
A royal source has said she was "doing well" but there has been no further information about her since she was admitted to the London Clinic last week.
She was due to spend up to two weeks in hospital and is unlikely to return to public duties until after Easter. Her husband Prince William, the heir-to-the-throne, has postponed his engagements to look after their three children, Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5.
Meanwhile, Duchess of York Sarah Ferguson, the ex-wife of Charles' younger brother Prince Andrew, said on Monday she was in shock after being diagnosed with a malignant form of skin cancer.
It was the second cancer diagnosis for the duchess, often just referred to as "Fergie", after she underwent a mastectomy and reconstructive surgery following the discovery she had breast cancer last summer.
