Philip, the husband of Queen Elizabeth II, was admitted to London's King Edward VII Hospital on Tuesday night, officials said. He was expected to remain there for several days of rest and observation.
King Edward VII Hospital is a private facility that's treated the British royal family for decades.
The duke of Edinburgh, who will turn 100 years old in June, celebrated his 73rd anniversary with Queen Elizabeth II in November. Both received a COVID-19 vaccine in January.
Philip, who retired from public duties in 2017, has lived with the queen at Windsor Castle since the pandemic started. He was last seen in public last summer while transferring the role as colonel-in-chief of The Rifles to his daughter-in-law, Camilla, duchess of Cornwall, who is married to Prince Charles.
Philip spent four days at King Edward VII Hospital in 2019 to treat what Buckingham Palace said was an unspecified "pre-existing condition."
He was also involved in a car accident in January 2019, but didn't require hospitalization. He did, however, surrender his driver's license as a result of the crash.