The incident happened around 9:30 a.m. on a southbound three train at 96th Street and Broadway on the Upper West Side.
He was evaluated by EMS.
Details of his condition are unknown.
The suspect fled from the station after the attack.
Police say they are looking for a man wearing a grey sweatshirt, grey pants and blue shirt.
MTA Spokesperson Aaron Donovan released a statement on the assault.
"Incidents like this again point to the need for a surge in mental health resources and uniformed officers into the system. We continue to urge City Hall to do more to address these ongoing issues, and to be transparent with the public about how many officers are on patrol in the subway system."
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