The girl, Maëlys De Araujo, was last seen in a children’s room at the venue in Pont-de-Beauvoisin at about 3am on Sunday.
Extensive searches of the area around the wedding hall using a helicopter and police dogs have thus far failed to find a trace of the girl, who was wearing a sleeveless white dress.
Police have already interviewed most of the 180 guests attending the wedding.
The Portuguese State Secretary for Communities is monitoring the situation and is said to have asked the Portuguese consulate in Lyon for more information.
SIC news reports that the Public Prosecutor in Bourgoin-Jallieu, in south-eastern France, said it had opened an investigation into the possible kidnapping of young Maëlys, but was not dismissing any possibilities.