A perv masturbated in front of a 20-year-old woman on the subway in Union Square and followed her onto the platform to put his privates on her, cops said Wednesday.

The encounter started just before 3:30 p.m. June 27 while the two were riding a southbound R train at the Union Square station, according to the NYPD.

The creep chased the woman out of the train car, pulled up her skirt and put his genitals on her back while she walked up the stairs at the station, police said. The creep took off through the turnstiles heading to the streets.

The NYPD released a photo of the man Wednesday and asked for the public’s help to identify him.