A teenage girl was groped in a Union Square subway station — but turned the tables on her attacker by following him onto a train as he fled and snapping his picture with her cell phone, police said.

The 19-year-old victim was waiting for the downtown No. 6 train at 9:30 a.m. Oct. 23 in the busy Manhattan station when the sicko lifted up her dress and groped her buttocks and crotch and then fled onto a train, authorities said.

The victim shrewdly got onto the train with him and snapped a picture of the pervert, which she gave to police.

Cops are hoping the public can identify the assailant using the shot from the young woman’s phone.