According to cops, Scout Willis was drinking an 8 oz. Pakistani beer in Union Square Monday night ... and when an officer approached her she produced an ID with the name Katherine Kelly.
After further questioning she 'fessed up her real name is Scout Willis and reportedly said, "The first ID isn't mine. My friend gave it to me."
Scout's bust was first reported by the NY Daily News.
Scout, who's currently attending Brown University, was booked for criminal impersonation and open container ... both misdemeanors. She was released without bail early Tuesday morning.