A pervert exposed himself to a 63-year-old woman inside a Union Square Park bathroom on Sunday afternoon and demanded a sex act — but she bit his hand, forcing him to flee, police sources said.

The woman was inside a stall at the public restroom at the northern end of the park about 1:15 p.m when the man, in his 30s, barged open the door and demanded oral sex, a police source said.

The victim bit the predator’s left hand, and the creep took off running, cops said.

The assailant has short hair and a full beard and was last seen wearing a green camouflage jacket.

The woman was taken to Beth Israel Hospital to be checked out.