According to the passenger, who was not named, the 62-year-old was visibly inebriated and tried to stand up before take-off, when passengers are asked to wear seatbelts, declaring: "I want to piss, I want to piss."
When a stewardess asked him to sit and wait 15 minutes until the jet took off and reached cruising altitude, he said he could not wait. "And there and then he stood up and did it on the floor," she told the French broadcaster.
"We could see he had been drinking. The stewardess was dumbfounded," she added. "No-one said anything. It all happened with courtesy. Mr Depardieu sat back down and the plane returned to the parking area to be cleaned."
The spokeswoman said the incident had taken place on Tuesday evening on board flight AF5010. She did not say whether the company intended to take further action against the popular French star.
According to a Paris Airports website, the Dublin route is served by an Avro RJ-85, a regional jet that can carry up to 111 passengers and is routinely equipped with onboard toilets both fore and aft.
Depardieu is perhaps the best known face in French cinema, having appeared in almost 200 films, and is known abroad as the star of the 1990 literary epic Cyrano de Bergerac and the US romantic comedy Green Card.
He is also a winemaker and in 1990 was convicted of drunk driving.