Police say it happened at about 8 o'clock Thursday morning on a Manhattan bound #7 train at the 74th Street and Roosevelt Avenue subway station.
According to investigators, a male touched a 16 year old female over her clothing in her groin and buttocks area then fled on foot.
The girl was not hurt.
A photo has been released of the suspect, who is described as a Hispanic male, 30-35, 5'9", last seen wearing a tan baseball cap, black hooded sweatshirt, a red turtleneck underneath and sunglasses.
Anyone with information is asked to call Crime Stoppers at 1-800-577-TIPS (8477).
The public can also submit their tips by logging onto the Crime Stoppers website at http://WWW.NYPDCRIMESTOPPERS.COM or by texting their tips to 274637 (CRIMES) then enter TIP577.