The fire was reported at 7:29 a.m. at a Salvation Army at 702 W. Taylor St.
Crews arrived to find thick smoke coming from the building's loading dock area. Dozens of mattresses were burning and firefighters called a second alarm as a precaution, Capt. Charles Rangel said.
The fire was contained at about 8:10 a.m.
Some employees who were inside the building when the fire started were evacuated but no injuries were reported, he said.
The cause of the fire has not yet been determined.