The King was moved aside by his security moments after they saw the egg thrown at him
An egg was thrown at the King during a visit to Luton town centre, said police.
King Charles was taking part in a walkabout during his visit to the town on Tuesday, greeting crowds in St George's Square.
He was temporarily steered away from the crowds outside Luton Town Hall by his security staff shortly after arriving.
He then resumed shaking hands with members of the public after being moved to a different area.
A man in his 20s has been arrested on suspicion of common assault and was in custody for questioning, said Bedfordshire Police.
The egg is thought to have been thrown in the direction of the monarch.
He had three engagements during his visit to Luton, visiting the town hall, a newly-built Sikh temple and a new shuttle at Luton Airport.
He met council officials and volunteers at the town hall, emerging on the building's balcony to wave to the crowds which had gathered below.
On a visit to Yorkshire last month, eggs were thrown at the King and Queen Consort as they walked in Leeds.
A 23-year-old man was later arrested and released on bail.