He was rushed to Northwestern Memorial Hospital in serious condition.
Police shut down service on the Green and Orange lines as they investigated the incident, which took place at Roosevelt and State just after 7 p.m.
Other commuters were injured as they tried to scramble for safety after the shots rang out.
"I was going up the escalator to go to the Orange Line and we heard some shots up there and everybody started screaming and running down the escalator," said Judith Nieves, witness.
Nieves showed ABC7 the bruises she sustained minutes after being trampled by a crowd of people trying to escape the CTA's Green and Orange line platform at Roosevelt after the shooting.
"We were trying to get up but every time we tried to get up people kept running on top of us," said Nieves.
Officers were seen taking away two men in handcuffs. Police said no arrests had been made, however.
The motive for the shooting is not known at this time.
A spokesperson for the CTA says the incident shut down power to the tracks for about 20 minutes as it was initially believed that the weapon may have been thrown onto them. By 7:30 p.m. power was turned back on, but trains continued to bypass the Roosevelt stop for an hour longer as police continued their investigation on the platform.