The Duke of Edinburgh was taken to the King Edward VII hospital in central London yesterday (June 4) due to a bladder infection.
The British royal will stay at the hospital for a number of days as a "precautionary measure".
It is currently unclear what caused the illness, but he is expected to make a full recovery after a course of antibiotics, according to BBC News.
The 90-year-old is said to be upset about missing the rest of the Diamond Jubilee celebrations, including last night's (June 4) concert at Buckingham Palace.
The prince previously underwent treatment for a blocked coronary artery in December 2011, but is otherwise said to be in good health for his age.
Earlier today, the Queen attended the National Service of Thanksgiving for her Diamond Jubilee at St Paul's Cathedral.