Police evacuated the eatery in Marseille and firefighters also attended the incident.
The siege is reported to have started at around 3.30pm.
One picture of the incident shows two fire vehicles and four firefighters outside the building.
And another photo shows more than 10 people standing outside the McDonalds.
The siege is reported to have ended with the man leaving the toilet.
The man is reported not to have been arrested by police.
There are no reports of any injuries at the scene.