The French airline said the child, who travelled with an unnamed woman, on Air France Flight 1891 from Istanbul to Paris, did not have a valid ticket.
According to Metro, a fellow passenger said he knew something was wrong when a carry-on bag started moving by itself.
Flight attendants are then reported to have opened the bag and discovered a baby - thought to be around 1-2 years old - inside.
Air France said it requested "the presence of French authorities on the flight's arrival" to deal with the matter.
Investigators are now trying to determine why the woman decided to sneak the baby on board the flight.