The fire broke out at a building in the North Austin neighborhood's 1200 block of North Lavergne Avenue.
A firefighter, a man in his 30s, was transported to West Suburban Medical Center in serious condition, the Chicago Fire Department said. He was taken to Loyola Medical for further treatment, where he is listed in critical condition.
A mayday call was issued as a precaution. The fire has since been extinguished, CFD said.
Officials did not elaborate on the firefighter's injuries and did not immediately provide further information.