The incident happened just after midnight on Sunday, March, 23, at the N line station at 62nd Street and 8th Avenue in Sunset Park.
Authorities say the suspect grabbed the 17-year-old victim's buttocks before fleeing on a the train.
The suspect is described as a light-skinned Hispanic male, approximately 30 years of age, standing about 5-foot-8 and weighing about 160 pounds.
He was last seen wearing a blue denim vest with a green hood and green sneakers.
Anyone with information in regards to this incident is asked to call Crime stoppers at 1-800-577-TIPS (8477).