A 28-year-old man has been charged with a public order offence after an egg was allegedly thrown at the King.
The incident took place during a walkabout in Luton town centre on Tuesday.
The King was temporarily steered away from crowds outside the town hall by security staff but continued to shake hands with the public after being moved to a different area.
In a statement, Bedfordshire Police said: "A 28-year-old man has been charged in relation to an incident which occurred during His Majesty King Charles III's visit to Luton yesterday.
"The man was charged under section five of the Public Order Act and has been bailed to appear at Luton Magistrates' Court on 9 January 2023."
The suspect's name has not been revealed by the force.
A man was also arrested last month for throwing an egg at the King in York.
At the start of yesterday's visit, the King met representatives from the Royal British Legion, the Ghana Society, Luton Town's football academy, veterans and cadets.
He later paid a visit to the Guru Nanak Gurdwara in the town and met volunteers at the Luton Sikh Soup Kitchen Stand.
The monarch wore a Ramaal headscarf for the visit to the new temple and bowed in front of the Sikh holy book, Guru Granth Sahib.
He also rode on Luton's cable-drawn Dart mass passenger transit system, which links the train station to the airport.
It comes as the Royal Family is braced for the release of Harry and Meghan's contentious Netflix docuseries on Thursday.