The attack happened just before 4 p.m. at the 57th Street and 7th Avenue station.
Police say a 40-year-old man was riding aboard a southbound Q train when he was slashed in the shoulder by another man.
Police are searching for the suspect who fled on foot.
The victim was taken to Bellevue Hospital with minor injuries.
It's unknown what, if anything, led up to the attack.
The investigation is ongoing.
This is breaking news. This story will be updated as more information becomes available.
----------
* Get Eyewitness News Delivered
* More Manhattan news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube