The four young women were in Marseille traveling while studying overseas.
A 41-year-old man sprayed them with hydrochloric acid from a spray canister which is available in DIY shops.
The spokeswoman for the Marseille prosecutor's office said the suspect did not make any extremist threats or declarations during the attack. He said the USA consulate in Marseille is in contact with investigators.
It's unclear what sparked the attack, which left two of the tourists with facial injuries and one with a possible eye injury.
The US embassy in Paris said diplomatic staff at the consulate in Marseille are in contact with French authorities. The other two were in shock.
Fourteen firefighters arrived to the station four emergency vehicles after the incident at 11am.
Marseille is a port city in southern France that is closer to Barcelona than Paris. Police said the attack was not terror-related. The alarming rise comes amid a clampdown on weapons and fears of a frightening new crime fad involving teenage motorbike thieves using corrosive substances, in part because they are relatively easy to obtain, The Washington Post reported last month.
The attacker did not shout any slogans to suggest the attack was linked to extreme Islamism, according to reports. All four were treated at a hospital Sunday.