Investigators say the suspect grabbed and sexually assaulted the victim as she entered the B and C line station on 135th Street.
The incident happened around 5:30 a.m. Thursday.
The woman was treated at the hospital.
The suspect is described as being 20 to 25 years old, standing about 5-foot-9.
Anyone with information is asked to call Crime Stoppers at 1-800-577-TIPS (8477).
Get Eyewitness News Delivered