The assailant, a 41-year-old woman, was quickly arrested in the Mediterranean port city of Marseille.
Two of the female tourists suffered facial injuries during the late morning attack at Marseille's Saint Charles train station and one of the two also had a possible eye injury, a spokeswoman for the Marseille prosecutor's office told The Associated Press in a phone call.
French authorities are not investigating the acid attack as an act of terrorism for the time being.
The suspect has "a psychiatric history", a spokeswoman for the police prefecture in Marseille said.
La Provence, the main local newspaper, quoted police sources as saying that after the attack, the suspect had displayed pictures of herself with burns on her body. Two were hospitalized for shock.
The spokesman says USA authorities in France are not immediately commenting further on what happened to protect the privacy of the American tourists, all women in their 20s. The two women reportedly sprayed with acid remain in hospital.
Four American women were attacked with acid outside a French train station today.
French reports say the incident is not being treated as terror-related and witnesses said no slogans were shouted during the attack.