A subway perv was masturbating in front of a 33-year-old woman on the 1 train last weekend when she quickly snapped a picture before he ran away, cops said.

The man was riding an uptown 1 train near Chambers Street around 9 p.m. when he exposed himself, according to police.

When he realized that the woman had taken his picture, the perv zipped himself up and exited the train at Franklin Avenue.

The suspect was last seen wearing a knit cap, white sneakers and a blue hooded sweatshirt.