The incident happened on line 5 of the Paris Metro in front of shocked passengers at rush hour on Tuesday evening.
According to reports, a man who was smoking a cigarette attacked a passenger who had remarked that lighting up was illegal on the Metro.
After an initial scuffle between the pair the attacker pulled out a box cutter and struck him several times.
The victim was left seriously injured and two other passengers were reportedly lightly injured in the melee.
The attacker fled when the Metro pulled into the station Bréguet-Sabin in the 11th arrondissement of the city.
Several police units rushed to the scene but were unable to track him down.
Metro services on line 5 between Bastille and Republique were suspended between 6pm and 7.30pm on Tuesday evening.
It turns out the victim worked for Paris transport authority RATP although he was off duty at the time.
He was described as seriously injured but was not in a critical state.