Police are looking for a man who masturbated on a busy Brooklyn subway platform and then followed a 51-year-old woman onto a train and groped her.

The victim was waiting on the B/Q train platform at the Atlantic Avenue station when the creep, began to fondle himself in front of her Aug. 7 about 6:20 p.m., cops said.

The woman walked away and boarded the train, but the pervert hopped aboard with her and grabbed her behind.

The victim managed to snap a picture of the deviant with her cellphone and handed it over to authorities.