The shocking incident took place at 10.30 am on Tuesday at the Dupleix Metro station in the wealthy 15th arrondissement in the south of the city.
According to Metronews which revealed the incident, CCTV images showed that the young art student was initially followed through the Metro station by her attacker.
As she stepped onto the escalator at the station “the man went behind her, lowered his trousers, before penetrating her using his fingers," a police source told BFM TV.
She cried out and her screams put off the attacker and he fled the scene. According to reports a passerby came to aid before calling police and paramedics.
She was taken to the Saint-Joseph hospital in the 14th arrondissement suffering from shock. She later filed an official complaint with police for the sexual assault.
"The investigation is being taken very seriously," a source close to the case told Metronews, while an unnamed police source said "There are no doubts about the veracity of the facts."
The incident has been classed as "rape” in France, becasue the definition of the crime includes penetration by digits, rather than just the penis, as is the case in the UK.
Her attacker has not been arrested and Paris's transport police are in charge of the investigation.
Gail Denicola, communications director at the American University of Paris told The Local that new American students are warned each year about the potential dangers of sexual harassment on the Metro.
"We tell students to be particularly aware on the Paris Metro and encourage them to take taxis at night," she said.
"We tell them to pay attention to their surroundings, no matter what time or day it is and to avoid being alone on the platform," she said.
"Stop - That's Enough!"
The incident comes as authorities in France are making efforts to fight sexual harassment on the Paris Metro.
A survey in April last year by the High Council for Equality between Women and Men (HCEfh) found that a full 100 percent of the 600 women in Seine-Saint-Denis and Essonne, two areas in the outer suburbs of Paris, said they had experienced at least some form of gender-based sexual harassment in their life while riding the train.
The government responded to the shocking results by launching a campaign under the title “Stop – That's Enough”.
The awareness campaign was launched together with rail operator SNCF and Paris transport chiefs RATP.
It saw flyers handed out and a set of posters put up in key places around the capital, encouraging victims and witnesses to speak out with confidence about sexual harassment.
The campaign also aims to remind the culprits that sexual harassment is punishable by law and groping can lead to five years in prison.
The picture below shows one such poster, and represents a Metro line where each stop is labelled with comments from an increasingly aggressive harasser. The comments escalate from "Mademoiselle" and "You're charming" to "I'm going to grab you".
By the end of the line, the response comes: "Stop - that's enough!"