Video shows the attacker throwing the liquid in the 43-year-old woman's face, hitting her in the arm, hair, and eye.
It happened Wednesday at 11:07 a.m. at the 3rd Avenue and 138th Street subway station.
It's not clear what was in the cup.
The MTA worker was treated for pain and irritation in her eye.
The man took off after the attack.
He is described as having a dark complexion and was last seen wearing a blue jacket, black hooded sweatshirt and light colored backpack.
Anyone with information in regard to this incident is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).
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