A spokesman said the airline is “very concerned” after a picture was circulated online of the incident, which occurred as passengers waited for a delayed flight to London Luton.
Arabella Arkwright, who took the photograph, described the moment a man holding a nine-month-old baby was hit on Saturday.
“The man with the baby... went over and talked to the Frenchman and the Frenchman didn’t reply, he just smiled and smirked and then whacked this guy on the left side of his face,” she told BBC Radio 5.
She said her 6ft 4in husband pulled the man off the apparent victim and “frogmarched” him away before police were called.
Both men were taken away, but fellow passengers demanded the man with the baby was allowed to board his flight, Ms Arkwright said, adding: “When he arrived on the plane, all the plane clapped him.”
An easyJet spokesman said the person in the photograph is believed to work for Nice Airport’s special assistance provider Samsic.
A statement said: “EasyJet is very concerned to see this picture and can confirm the person in the photo is not an easyJet member of staff and they do not work for easyJet’s ground handling agents in Nice.
“We are urgently taking this up with Nice Airport and their special assistance provider Samsic, who we understand the person photographed works for.”
The airline apologised for the delay to flight EZY2122 from Nice to London because of a “technical issue with the aircraft”.
The statement released on Sunday said: “Passengers were provided with updates and refreshment vouchers during this time and the flight landed in London Luton last night.
“The safety and wellbeing of our passengers and crew is always easyJet’s highest priority.
“EasyJet sincerely apologises for the delay and thanks passengers for their patience.”