The firefighter was on the roof of a three-story building in the 800 block of North Hamlin Avenue in Humboldt Park when he fell through to the basement, Chicago Fire Department officials said.
The injury happened while CFD crews were responding to a fire that broke out just after 6 p.m.
The injured firefighter was taken to Stroger Hospital for serious fall injuries. A CFD spokesperson said Monday morning the firefighter is in stable condition with non-life threatening injuries.
The fire department said all residents were able to evacuate safely on their own before crews arrived.
Eight people who lived in the building were uninjured, but they have been displaced, CFD officials said. The Red Cross responded to assist them.
Chicago fire officials said they dealt with hoarding conditions on the second floor, so there was a lot of stuff inside making the fire worse.
Witness April Rimmer was down the street when the fire started, and she called 911.
"It was real crazy and everyone just screaming, 'it's a fire, it's a fire,' but no one pulled out their phone to call 911, so that was my first thought," Rimmer said. "And I called 911 as soon as possible. This is sad... so sad."
ABC7 was also told there was a family of migrants from Ecuador living in the building. They said they've been there about six months now and just lost everything.
The cause of the fire was not immediately known. No further information was available.
This is a breaking news story. Check back with ABC7 for updates.