The incident occurred at the State and Lake stop at about 2:44 a.m., police said.
A man was on the platform with a group of people when police said he started to argue with someone in the group.
Someone in the group produced a knife and stabbed the man in the chest, police said. The victim was transported to Northwestern Memorial Hospital in fair condition.
Police said the incident was drug-related and Area Three detectives are investigating. A possible suspect is in custody, police said.