Cops are looking for a well-dressed pervert who stroked the knee of a teenage straphanger on Thursday.

The 15-year-old was riding a Brooklyn bound B train around 4:00 p.m. when the balding man in a black dress jacket and striped tie took the seat next to her at 34th Street and began caressing her left knee.

The the teenager was able to snap a picture of the offender.

She exited the train at Newkirk Plaza, where the creep also got off. He did not follow her, police said..