The incident occurred around 4:55 a.m. at the Blue Light bar, 3251 North Western Avenue, between Belmont and Clybourn.
The woman was cut with a razor, said one report.
The offender was described as a tall, Hispanic male dressed in all black clothing.
According to eyewitnesses, the assailant fled the scene eastbound on School Street in a black 2012 Dodge Challenger.
During the last update, authorities said the victim lost a lot of blood and one person was arrested in connection with the stabbing.