The fire began about 3 a.m. CDT Thursday at one of the 12 to 15 structures at the shuttered prison, said Deputy Fire Chief Ray Randich.
He said the building was 90 years old and was previously used as a mattress factory but more recently served as a training facility, WLS-TV, Chicago, reported.
None of the more iconic buildings at the complex featured in television and movies were damaged, fire officials said. The facility was used in the opening of 1980's "The Blues Brothers."
Randich said firefighters initially had difficulty getting to the buildings because gates were welded shut and locked since the prison was shut down in 2002.
"There have been reports on the police side that people have gotten into the facility and have been, you know, doing whatever they do when they get inside these places, but for the most part, from our point of it, the fire department could not get access, " he said. "We had some of their keys, the Stateville Correctional Center oversees this property now, but again, with nothing being opened or moved since 2002, we basically, you know, could not get any of those gates to work as we had wanted to. The gate was welded shut."
The Joliet prison opened in 1858 and closed in 2002. Since its closure, squatters have been reported at the site, WLS said, but Illinois Department of Corrections Spokesman Tom Shaer told the Chicago Tribune it was nearly impossible for squatters to enter the buildings. The cause of the fire was not determined.