The attack was carried out at Saint Charles station in Marseille and the man reportedly yelled Allahu Akhbar before lunging at passengers.
French police say the incident is likely a "terrorist act" and are warning people to avoid the area as they carry out an operation.
French Interior Minister Gérard Collomb is now travelling to the scene.
The mayor of Marseilles reportedly said "the number of victims could have been higher" had it not been for the army.
According to Sky News A police source said the attacker killed one man on a platform and seriously injured another person. The Sun reports Oliver de Mazieres, a local official confirmed "two victims have been stabbed to death". According to the Mail on Sunday Both victims were women, with one left with a 'sliced neck'.