The incident briefly overshadowed Macron's visit to the International Catering, Hotel and Food Show.
An egg was thrown at French President Emmanuel Macron during his visit to a restaurant trade fair in Lyon.
The man who threw the egg was immediately removed from the room after the incident and detained for "assault on a person in a position of public authority".
The egg bounced off Macron's shoulder without breaking as he was walking the aisles on Monday.
"If he has something to say to me, let him come," the French President said in response.
On Tuesday, the Lyon public prosecutor's office said that the suspect, a 19-year-old student, had been placed in a psychiatric facility.
In a statement, prosecutors said the man "was no longer capable of judgement".
The incident happened during Macron's visit to the stands of the International Catering, Hotel, and Food Show (SIRHA).
In June, Macron was also hit in the face by a man on the sidelines of a trip to Tain-L'Hermitage.
The French President was warmly greeted by restaurant professionals at the trade fair in Lyon, with many thanking Macron's government for providing state aid to the industry during the COVID-19 pandemic.
On Monday, Macron also announced a forthcoming tax exemption for tips paid by bank card.