The 26-year-old was on the Kedzie-Homan platform in the 500-block of South Kedzie Avenue about 5 a.m. and got into a verbal altercation with a female suspect, police said.
The suspect pulled out a knife and stabbed the man in the right side of the chest before fleeing, CPD said.
RELATED: Chicago police add new CTA train patrols, K-9 units to return; non-violence groups offering support
The man was cut and taken to Mount Sinai Hospital in fair condition.
No one was in custody later Thursday morning, and Area Four detectives are investigating.
O'Hare-bound Blue Line trains were running with some delays after the incident, the CTA tweeted just before 5 a.m.