The victim, identified in French media as a 20-year-old homeless man, was said to have been sprayed with an unknown substance during a confrontation at Bastille station on Friday morning.
He is being treated for major burns to the face and hands after being removed from the station by emergency services and rushed to hospital.
Reports suggested a fight had broken out at the station between the victim and an “aggressor” at around 6.30am
“A rag soaked with a liquid that could be acid was found” at the scene of the incident, a police source told Le Parisien.
Police said inquiries were underway, adding the perpetrator was still at large.
Investigators have ruled out any links between the incident and a similar attack on Wednesday, when a man and a woman riding on the metro were burned by an “unknown liquid”.
In September 2017, two American college students were injured in a hydrochloric acid attack by a woman at a train station in Marseille, southern France.
However, authorities said attacks in which corrosive liquids are used as weapons were still relatively uncommon in Paris.
“We do not deal with this type of injury more than a dozen times a year,” a Paris Fire Brigade captain told Le Figaro.
Additional reporting by AP