First responders arrived to the South Shore apartment around 1:59 a.m. in the 2300 block of E. 79th Street on Sunday morning.
Chicago police said a 38-year-old man was found inside the apartment building. He had multiple burns throughout his body.
Chicago firefighters took the man to the hospital where he was listed under critical condition.
No other injures were reported. The fire was out by 4:45 a.m. according to police.
Chicago police said the building fire was accidental.