In a video circulating on social media, Mr Macron was seen walking towards a fence during his trip to Tain-l'Hermitage on the outskirts of Valence.
The man washed Mr Macron's face with a handkerchief before officers rushed to his aid. The president was then fired.
French media say two people have been arrested for allegedly assaulting the president.
As the president was slapped, some people were heard shouting "God bless Macron's ideology".
W ane is to protect the President macro?
French leaders are protected by GSPR.
They were formed in 1983 and are reported to have a 77-member team that included men and women defending Mr Macron during rallies.
French television station BFM said officials were inspecting the premises before the president's visit. Armed officials are being sent to protect the president during his tour.
What are people saying ?
Politicians immediately denounced the move.
Prime Minister Jean Castex told parliament shortly afterwards that although democracy meant debate and disagreement it should be "but it should not be full of violence and insults and both".
Communist Party leader Jean-Luc Mélenchon posted a Twitter message "supporting the president".
Officials took to the streets of Valence shortly after the march, according to a tweet from a French journalist.
President Macron is on a tour of France and visits a hotel art school in Tain-l'Hermitage.
Officials say he is scheduled to continue his visit on Tuesday, where he will visit a vocational training center set up 25 to 30 years ago.
The president's visit also comes in the evening of a relaxation of the country's cholera control rules where restaurants and bars will be reopened after seven weeks of closure.