A community alert said the suspect waits for a train to approach a stop, rips the necklace off the victim's neck and takes off running through the opening doors.
Police said the suspect has stolen three necklaces in the last three weeks.
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The most recent robbery occurred on a Blue Line train near the Division stop in the 1200-block of North Milwaukee Avenue around 6:50 a.m. Wednesday.
Another took place on a Red Line train near the Cermak-Chinatown stop in the 100-block of West Cermak Road around 7:05 a.m. on Aug. 3.
The first victim was robbed on a Blue Line train near the UIC-Halsted stop in the 800-block of West Harrison Street around 3:30 p.m. on July 27.
CTA surveillance photos show the suspect is a bald African American man between 20 and 30 years old with a dark complexion. He is between 5 ft. 3 in. and 5 ft. 6 in. tall and weighs about 145-155 pounds. He was wearing a light-colored baseball cap in two of the photos.
Police urge CTA riders to be alert and to keep valuable jewelry in a purse or bag during their commute.
Anyone with information regarding these robberies should contact Area Central detectives at (312) 747-8384.