Police were seen investigating at the Jackson Blue Line station in the Loop.
Passengers noticed that an older man was unresponsive, and officers responded to a call for service around 1 a.m., CPD said.
The Chicago Fire Department also responded to the scene, and the man was pronounced dead. His identity was not immediately released.
Area Three detectives are conducting a death investigation pending autopsy results.
The CTA said Blue Line trains were running with significant delays and instructed riders to board trains on the O'Hare-bound side at the LaSalle, Monroe, Washington, Clark/Lake, and Grand stops.
Authorities did not immediately provide further information.