The man was on the train about 3:30 a.m. near the Chicago station when someone walked up to him, hit him on the head with a bottle and stole his wallet and money, according to CPD.
The man suffered a cut on top of his head and was taken to Northwestern Memorial Hospital in good condition.
RELATED: South Loop safety meeting zeros in on CTA crime, which is up compared to last year
No one is in custody, and Area Three detectives are investigating.
Chicago police did not immediately provide any information about a possible suspect.
CTA riders and South Loop residents held a meeting last week to express their concern about an increase in crime on the transit system so far this year.