The two tourists have suffered burns and have been admitted to hospital. She did not release more details about the victims, including where in the United States the tourists live.
The prosecutor's office could not be reached for comment but told France 3 that one of the women had been hit in the eye with acid and had trouble seeing.
A police spokeswoman told USA Today that the suspect did not yell any out any terror-linked threats.
French authorities said they do not believe the attack is terrorism. Authorities said two French nationals were arrested in Marseille before they were able to carry out what Paris prosecutor Francois Molins called an "imminent, violent action".
The four women were studying overseas, three of them at the college's Paris program.
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The US President tweeted a response this morning, taking aim at terrorists who he described as "sick and demented". Trump concluded, "Must be proactive & nasty!" Police advised people to avoid the area in southwest London.
Two of the Americans were "slightly injured" with acid but did not require emergency medical treatment from medics at the scene, the spokeswoman said.
Four American women were attacked with acid outside a French train station today.
The spokesman says US authorities in France are not immediately commenting further on what happened to protect the privacy of the American tourists, all women in their 20s.
She said a 41 -year-old female suspect has been arrested.
The port city of Marseille, about 500 miles southeast of Paris, has seen at least two other attacks in recent months.