Buckingham Palace said on Wednesday that the 91-year-old Duke of Edinburgh was transported to Aberdeen Royal Infirmary. However, it did not give details of his condition.
Philip was hospitalized for five days in June with a bladder infection .
He later resumed his royal duties, attending the opening ceremony of the Olympics on July 27 and carrying out engagements on the Isle of Wight on Monday.
Philip married the queen in 1947. The former naval officer's health remained robust through his 80s, but he has appeared frailer in recent years.
When he turned 90, he said he was "winding down," and he has cut back on official duties. But he still carries out some engagements, both with his wife and on his own.
Philip spent four nights in the hospital over Christmas after suffering chest pains and underwent a successful coronary stent procedure.
The Associated Press contributed to this report.