Officer Preston Ross Jr. was in uniform working near the CTA Blue Line station at Grand and Milwaukee when he reportedly experienced problems breathing.
Ross was rushed to a hospital where he died. The police department said he may have suffered a heart attack.
Police are awaiting autopsy results to determine exactly how he died.
"It is with profound sadness that I announce that the Chicago Police Department has lost one of its own this morning, Officer Preston Ross Jr.," police Supt. Garry McCarthy said in a statement. "On behalf of the entire Department, I offer my sincere condolences to Officer Ross' family, friends and colleagues. We will keep them in our thoughts and prayers during this difficult time."