Police say it happened on the 1/2/3 train platform just before 7 p.m. Friday.
Citizen App video showed the scene inside the subway station.
EMS crews wrapped the 35-year-old victim's wound with bandages before taking him to Bellevue Hospital in stable condition.
No arrests have been made and the suspect remains on the loose.
The circumstances surrounding the slashing are unknown.
The investigation is ongoing.
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