The attacker was "neutralized" by police, interior minister Gerald Darmanin said on Twitter, without going into further detail, Reuters reported.
The attack happened around 0640 CET (0540 GMT) inside the train station, radio franceinfo said, quoting rail operator SNCF.
The station is one of the busiest in Europe and a major link between Paris, London and the north of Europe.
Police have secured the area and trains were opertating normally, franceinfo reported.
BFM TV said police had used a gun to stop the man.