Department spokesman Will Knight says the firefighter's name isn't yet being released, pending notification of family members.
Authorities say the firefighter was an 18-year veteran of the department who died late Sunday after going into cardiac arrest.
Knight says the firefighter was cleaning his equipment after responding to a fire on Chicago's South Side when he began to feel short of breath. He later was pronounced dead at Little Company of Mary Hospital in Evergreen Park.
His death comes just days after the funeral of Capt. Herbert Johnson. He was a 32-year department veteran who suffered inhalation injuries while responding to an extra-alarm fire on Nov. 2.