The incident happened on Tuesday just after 8 a.m. The 17-year-old girl was sitting on a bench at the 176th St. station when the suspect sat next to her and snatched her necklace.
The teen tried to take her necklace back, but the suspect flashed a knife before running away.
The teen had minor injuries but did not go to the hospital.
Anyone with information is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477).